My son is 3, will be 4 in January. I'll be getting him started this winter.
Just going to start out pulling him around on flat-ground, getting used to being in a board and on snow. Then whenever he's comfortable, start letting him go down slight inclines, then slowly add in shifting weight edge to edge and getting some movement.
Whenever teaching the kids, just make sure they're having fun and you're not pushing them forward too far. They will get freaked out and turned off the sport if you're pushing them too far out of their comfort zone.
I've been doing about the same techniques with my son on his skateboard this summer and he's loving it. Started out just holding his hands and pulling him along the sidewalk, then ran alongside him going down small inclines, ran along side while he was rolling back and forth between wall of a mini-ramp, and he's now rolling down bank ramps without wanting to hold my hand. Just let them guide the progression and have fun and it will be all good.
A lot of people say that at 3 they won't have the muscles, balance, etc. While this is true, starting then will help build those traits earlier than if you wait for their natural development, at least in my opinion.