X-MenFilms.net is saying that Daniel Cudmore "IS" playing Colossus in X-Men 3. He is on set right now filming some of his parts...
Also, Vinnie Jones will be wearing a huge Juggernaut suit to make him as big as the Juggernaut. I'm sure this will be used in some of the close up shots, etc...., but that CGI will be used heavily for Juggernaut, Colossus, etc.etc. Especially in the action scenes!