In any combination of those classes, it's possible to be successful at most anything in the game. For the most difficult content, it will help to have either the Guard or Warden, but with enough practice, a Captain can tank successfully. There are situations where certain classes will make everything much easier, though, so it might be a good idea to incorporate all the alternate characters into your playing time, for practice and leveling. Flexibility is great.
[color=red]Work like no one is watching, dance like you don't need the money...[/color]