I did it with a MNK/NIN while I two-boxed RDM/BLM and WHM/SCH. I had no trouble sleeping them on RDM, overwriting with Sleep II and Repose. (And hot damn do I love Repose.) While not impossible, it was a bit of a challenge. (Especially at the end with the 4 BLM Quadavs.) I think that melee, sleeper, and healer is the best way to do it with 3 people, going in with more is not a bad idea.
Also, after the second cutscene with Gentle Tiger, speak to him again to get a title.
And holy crap, the animations that they used in both of the cutscenes were amazing.
http://wiki.bluegartr.com/Storm_on_the_Horizon
http://wiki.bluegartr.com/Fire_in_the_Hole