Hi Paul. Northstar has a group class that can be morning or afternoon or all day that is 3 year old specific called the Lucky Stars or something like that. I had just looked that up for a friend of mine and it that's the one I found that not only would take 3 year olds, but had a class just for 3 year olds.
There are also private lessons, and they had some kind of family lesson too.
Diamond Peak takes 3 year olds only in private lessons. I can't remember about the other resorts, like Alpine (may have 3-4 year old class), sorry.
If you end up in South Lake, let me know if I can do anything for you.
Have fun, the snow is AMAZING right now. Are you there this week? It's "peak" season until the 28th or so I think, so might be a tad more $$, like $10 or $20 per lesson.