If I was to play healbot for a raid I'd go with two holy paladins and a holy priest. The holy paladins are like 'fire and forget' MT healers while your priest can spot heal. Very powerful healing combo imho.

If I was to play DPS I'd look at how your guys are gonna contribute to the raid.

3 locks with improved shadow bolt would be sexy.

3 shadow priests all in different groups would be nice for everyone else.

3 fire mages could be fun because you could just turn one into the scorch mage so no one else is forced to do that.

3 shamans is awsome for all the reasons that everyone multiboxes shamans.