Re: Would you let your 3yo SON....
I wouldn't. I would tell him "you can do it when you are 8" (or pick an age). I think kids usually are able to understand that they can do activity x at age y.
Reason being, pierced ears on a boy do stand out as unusual. At 3, he can't understand that yet. And if he changes his mind and wants them gone, the holes will still be there, eventually scars but still visible. So it is a permanent decision to do something outside the mainstream, and 3 is too young to grasp what the implications are.
But if he turns 8 and still wants it, still just as much, then after 5 years it might be time to take the chance and let him do it.