I successful three-manned Drakkari Colossus / Elemental last night after many many wipes and messing with healing rotation. Here is what I did:

Phase 1 - Colossus Phase: I simply used autoshot and pet attack to do dps and just spot healed the tank as needed. This allowed me to conserve mana for the later phases

Phase 2 - Elemental: I moved my tank closer to my slaves and formed a tight triangle: spread out enough to keep everyone in chain heal range but out of range of each others ground debuff. I did not move my toons a single bit. I changed my healing rotation to use a /click method where I would randomly cast a chain heal on either the tank or my shaman. This was more than enough to keep everyone's health up through the group damage. I tried it with spamming chain heals just on tank and a try where I spammed chain heals on my healer and neither would work. The random target was what did it. I then went balls-to-walls on dps during this phase.

Phase 3 - back to Colossus: Tank grabbed aggro and again, i just used auto shot and pet attack while I healed up and got back up to full mana. I think at the start of this phase I nudged my toons a step or two to get out of the aoe damage.

Phase 4 - final phase: Moved back into the triangle formation, popped heroism, blew trinkets and all cooldowns and wtfpwn'ed the elemental until he was dead.