Is he limping? Does it still hurt? Sometimes a kid will have an itch and say it "hurts", it may mean absolutely nothing at all, in fact I know it doesn't. Even if his knee DID hurt for a while, he's a 3 year old boy, he will be bumping and bruising himself more and more as he gets older. Why would you even think that is cancer? Please don't google, I have almost destroyed my sanity with that. And don't let him know that you are concerned or worried about anything because you will pass that on to him (I know, I made my daughter a neurotic and now I can't "walk it back")
If he is warm it could be a little cold coming on, or a little winter virus, nothing to be concerned about at all.