"Dear Kate, here are some fun things to know about having a Third Child."

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The Duchess of Cambridge is now the mother of a ‘Third Child’. Congratulations, Kate.

I’ve been in your shoes for almost 10 years and I have some great news: third children raise themselves. They have to.

Their whole lives are like being in an episode of Survivor where the host keeps forgetting you’re even a contestant.

Oh that’s right! You’re here! What’s your name again? Carry on.

This is an actual text exchange I had with another mother just a few weeks ago that illustrates this point:

He can climb through the dog door and has the code to the gate.

See how easy it is to have a third child, Kate? You're going to bloody love it.

And more good news: you don't even have to have three children to parent as though you're a mother-of-three although admittedly, it's harder (make sure you mention this to Pippa and Meghan).

Third-child parents are the way we are by necessity. We have less time and give fewer shits. There's also an enormous freedom in learning that so much of the small stuff you sweated when you had kids one and two was essentially a waste of everyone's time - including your child's.

That enormous birthday party you threw for their first birthday? The one where they threw up on their 'friends' who, like the guest of honour, won't ever remember being there? That elaborate outfit you bought for your firstborn to wear on the drive home from the hospital? The paralysing anxiety you had over feeding them formula for the first time because you were terrified it could ruin their life? The way you insisted no precious child of yours would ever consume sugar or watch a screen until they were at least two? The hours you spent pureeing organic carrots and washing newborn clothes in organic soap flakes? The way you insisted you would never allow toy guns into your house under any circumstances?

So apparently this thing called "Third-Child Parenting" actually exists as a child rearing technique. I can't be bothered to Google it to find out much more because I have three children and I stopped reading parenting articles about two kids and 10 years ago. But I can roughly work out what it is and I'm all for it.

I have a friend who is trying for her third, and I was telling her how three is a great number because it means there's less attention for everyone which I think is a wholly positive thing. No human ever benefited from being treated as the centre of the universe for their entire childhood, whether they have siblings or not.

The best thing about your third child?  They have really low expectations for how you roll.

I find this enormously helpful. Because the parenting maths just doesn't work any other way. You simply do not have the same amount of time or energy to devote to parenting your third child as you did when you only had one or two. You don't have the same amount of time to do anything you used to do when you had fewer or no kids including having sex, cleaning your house, putting on makeup - or pants.

So the key to your well-being as a mother of three (or any) is to lower the bar for yourself. It makes parenting less stressful and kids more self-sufficient.

I'm very good at this.

Here are a few recent examples of third-child-style parenting from my life:

1: Forgetting to organise a birthday party.

I'm watching reality TV with my Third Child, who just turned nine. I know he just turned nine because one of the other mothers in his class emailed me a couple of weeks ago to ask whether he would be free on a certain date to attend her (firstborn) son's 9th birthday party. This date was two months away. It was an Avengers themed party and she would be making costumes for all the children who were attending.

I realised suddenly that my son's birthday was in 10 days and I'd organised nothing.

Cue: mad scramble to hastily organise laser-tag, the trusty fallback for every third-child birthday party.

Ever noticed the Duchess of Cambridge does a lot of kneeling? Here's why. Post continues after audio.

2: Not knowing everything about your Third Child's life.

So anyway, I'm lying in bed with Third Child and he's reading me a story (I've been reading stories to my kids for 18 years, I figure it'll do him good to practice his reading - #3rdchild!), and I sniff his hair, in that way you do with your kids. I like to sniff my kids very much. It bothers my 20 year old but the younger ones still let me. Third Child's hair didn't smell too bad but it didn't smell even remotely of shampoo.

Which made me wonder when he last washed it.

So I asked him.

And he couldn't remember. Over the next few minutes we established the following:

1. He had definitely washed it since we moved into our new house, which was 18 months ago.
2. He definitely washed it at some stage when he was seven. Which was two years ago.

This gave us quite a large window of time in which to guess when shampoo last touched his head. Anywhere from a week ago (highly unlikely) to more than a year ago.

This is third-child parenting. And it's awesome.

3: The tooth fairy.

Third Child lost a tooth a few weeks back. His third. Or maybe his second? Possibly his fourth. He'd swallowed the first one and I kind of lost track of what happened after that.

"Did the tooth fairy come?" I asked him, genuinely interested.

"Nah. I know it's just your parents."

Right. When my first child lost a tooth, the world stopped turning. Grandparents were called. 1800 photos were taken. There were wild celebrations. The tooth fairy left a trail of glitter from the window to his pillow, under which there was an elaborately written tooth fairy note (in glitter pen bought specially for the occasion) complete with short questionnaire ("How many times do you think this tooth has been brushed?"), flower petals from the garden, and a fresh $5 bill placed lovingly on a bed of tinsel.

Huge deal. Huge.

The first time he lost a tooth, Third Child didn't even bother to put it under his pillow, and honestly didn't seem fussed when I crept into his room at 6.30 a.m. the next morning (when I remembered I'd forgotten to leave any money).

"Hi Mum," came a voice from the dark.

"Oh, hey dude," I replied sheepishly.

"You've come to be the tooth fairy, right?"

"Yeah. I have."

And then I handed him $5 and we got on with our morning. Now we have it down to a highly efficient transaction. He loses a tooth, he brings me his wallet which he hands me along with the tooth and I fish out whatever change I can find. Sometimes I have to borrow money from his sister. Easy as.

Can't remember what life was like before you had kids? Here's a cheeky reminder. Post continues after video.

4: The things I 'forgot' to teach my Third Child.

The week my third child started school, it became apparent that I'd forgotten to teach him the alphabet. A few years later, I learned that he didn't know how to use a telephone or tie up his shoelaces. It happens.

With your first child you are obsessed with their 'firsts' and their milestones. First solids! First steps! First tooth! Each one is captured in hundreds of photos and videos and social posts. Hashtags are made along with an enormous fuss.

By the time two and three come along however, the novelty wears off and you're honestly just so busy getting through the day that it's easy to just miss those moments. This isn't ideal probably and no doubt it's grist for the therapy mill of your second and third-borns, but it's life. What can you do?

I found that the act of actually teaching my children life skills sort of slipped my mind by the time I had my third. Partly, I think, because I was just so worn down by 20 years of parenting, I sort of just lost my mojo. There's nothing like having one child who is old enough to have sex, drink alcohol and drive a car to make you forget that your work is not done because you still have smaller children who aren't quite that independent. However, he eventually learned all of those basic things and more.

And the great news is that benign neglect is a strong driver for resilience and resourcefulness. My Third Child is without doubt the most independent of my lot. He can even cook which is hugely helpful to his survival in a house where I live.

When his friends come over, he makes them toasted sandwiches and whatever other snacks they might want. He and his sister make their school lunches every day. Here he is making a toastie. And using a sharp knife. I would NEVER have let my firstborn do that.

Happily, my son has made friends with three other Third Children and their mothers and I all have #thirdchild shorthand. Recently, I went over to one of their houses to pick him up after a play date and the Mum said: "I dropped him home about an hour ago."

"Oh. I didn't notice he was there," I said. "Third child" she replied and we both nodded. I returned home to find him playing a video game, no doubt something inappropriate I would never have allowed his older siblings to play. Safe though.

So Kate, I hope you're feeling good about this third child. I think the biggest shock is from no babies to one. The jump from one to two is difficult but in different ways and the third? Well, it's undeniably hectic but it's also kind of freeing. You're more confident as a parent.  You're less scared of germs - and toy guns, sugar, formula, non-organic food or a break in routine. The switch to daylight savings (and back again) won't send you into a heightened state of anxiety. You know that the most important things are that your baby - and child - feels loved and safe and understood, even if you occasionally forget their name and let them crawl through the dog door.