For the case of X-Men: First Class
According to lee's Twitter page, due to the filming taking place
to far away he won't have a cameo in it.(Source:marvelousnews.com)
For the case of X2: X-Men United
Scooper 'Kailem' tells us that Stan Lee was interviewed on the
"Filmfreak" radio station when he mentioned he will not have a cameo
It seems that unlike the original movie, he isn't going to be turning
up anywhere in X2. When asked if he had a cameo in the sequel he
replied "No, no I don't, I don't think I'm gonna have cameos in the
sequels 'cause I think that gets to be a little much. I'm hoping I can
have a cameo in the first movie of each character." (Source:comicbookmovie.com)
And for X-Men Origins: Wolverine case ,Wolverine is one of the few characters not written by Lee.
So that's what i all got.