"That's because 3 is all nice and curvy while the others each have at least one straight line.

Our mind thinks it stands out. "

I'm deeply sceptical of that claim. If you ask people to pick a number from one to ten, they're more likely to pick seven than three, six, eight or nine, which are all round and curvy. And if you ask people to pick a number from one to fifty, they're most likely to pick thirty-seven.