Well for one combo champions are not reliable for MvPing, you might aswell asura..
Keep in mind that after every 'combo' your spirits are wasted and you need to recast them, that mixed with tanking the MvPing is really not efficient.
3 people can do a lot more than the 3 classes you suggested.. paladin is good for devo and tanking, but LK and Combo champs can be replaced with other more suitable ones.
IMO, the classes you should be focusing on:
Priest obv
Whitesmith
Sniper
Biochemist
Star glad
Paladin
Asura champ
Lord Knight (but outclassed by most of the other classes)
Prof (web mobs, swall.. etc)
Your main DPS is either WS or Biochem, sniper can take on quite a few MvP's too, Star glad with a swalltank can down a few too.
Paladin, priest, LK, prof are all tanks.. the reason LK is a tank is because of his high HP mod, and whatnot.. can also parry and BB while he's at it.