i would recommend 1 unholy/blood dk(blood up to runetap/imp runetap for more heals), 3 frost/blood(blood up to runetap/imp runetap for more heals) two hander dk spec as it works better given what talents we snag in blood tree, and a shaman healer or pally healer, as the dks should have most of the aoe healing covered and an all plate team is fun(you could theoretically spec the pally ret/holy with divine storm healing and spot healing when needed)

if you're only doing 3 i suggest doing 3 frost/blood dks Armory Moocrewa, Moocrewc, and Moocrewd on azgalor server. i run a team of 3 frost/blood dk and a feral druid tank and i mow thru stuff pretty well, and its easy to set up the macros and such