One of the most fascinating aspects of being a father of three is watching the shifting dynamics between two boys and a girl separated by little more than four years.

For sure, there’s some sibling rivalry. Isaac (six) and Toby (four) have their fair share of spats, usually over who wants one specific car out of the 9,420 they own between them. Or the iPad. Always the iPad. (God forbid I should ever want to use it!)

Equally there has long been an element of competition between Toby and 22-month-old Kara. All Toby has to do is even sit down next to Heather or I and Kara will come running over with a face of thunder, interpose herself and, if necessary – or even if unnecessary – throw a well-aimed set of talons at him.

But the one relationship that seems to transcend any real competition or territorialism is the one between oldest and youngest.

Kara has never shown any sign of jealousy towards Isaac. I’m not sure why exactly. He certainly has a more open, peaceful and attentive disposition than Toby – both of them are big on cuddles, which Toby dismisses as needless soppiness – and consciously takes pride in being the mature big brother. There’s probably a degree of tactical alignment in sidling up to her big brother too, creating a useful two-on-one when called on.

And there’s definitely a degree of hero-worship too. Kara watches Isaac like a hawk and wants to imitate everything he does. In her mind, she clearly thinks she’s six years old and can do anything her brothers can do. In return, Isaac gets a real kick out of adopting the ‘teacher’ role, patiently showing Kara how to do things. (To be fair, he does the same with Toby too, but his younger brother is less receptive to being bossed around taught.)

The dynamics never stop shifting, though. Kara is noticeably less violent competitive towards Toby than she was even a few weeks ago. And the relationship between the boys can turn on a sixpence, from joyful collaboration to petty bickering and back again, sometimes in the space of minutes.

But what’s best of all is those magic moments when all three of them are playing together. It doesn’t happen all the time (if only!) but it happens often enough. Sometimes the boys will build a den or concoct some elaborate role-playing game and go out of their way to include Kara (or, at least, not exclude her, which is often all she needs). Or, bound together by a common love – dancing and singing, picnic lunches, Top Gear – they will happily spend their time playing or sitting peaceably in their chairs as a threesome.

The ebb and flow of these interactions is all a new experience to both of us – Heather’s an only child, while the age gap between my brother and I is more significant at six years – but, rough or smooth, it’s fascinating to watch and be a part of.

The one thing we haven’t seen yet is all three of them ganging up on us. I’ve no idea how we’ll deal with that one. I might have to purchase that boxset of Outnumbered for hints and tips …