Our three year old consistently says "thank you". I think it's actually atypical, judging by his peers at preschool and our friends' kids. I think he picked it up because we're constantly emphasizing it with his older sibling, and him, too. And we're modeling it with each other and the kids.
That said, "please" is a bit slower in coming. He says it sometimes, but we're still hearing a demand/statement beginning with "I need" or "I want" more than a question beginning with "May I please have" or "Can I have . . . please." We usually just restate it for him and move on.
I think it takes A LOT of reinforcement for this to sink in. Lots and lots and lots and lots.
Meanwhile, he does say "excuse me" every time he burps or passes gas, which is HILARIOUSLY cute at his age.
