I've healed Garuda HM many times. The Sisters phase always comes down to the same problems:

1) All three mobs have frontal cleaves, and
2) Someone gets threat, then drags one or more mobs in front of all the DPS (and/or heals).

The appearance of the Sisters should be treated as a threat wipe. Don't Medica2 up front; leave Regen off the tank; appoint a DPS to act as the puller for the Green sister.

MT keeps Garuda, and has her faced away.
OT keeps Red, and has her faced away.
DPS takes Green; initial DPS tank has her faced away. Think "triangle"; Garuda and her Sisters go at the vertices, facing outward. Heals and DPS to the middle.

If you appoint a high-threat DPS to pick up a sister, they've got a risky job -- but heals will also know who to take care of, making the enterprise survivable. Try not to put all DPS in front of Green due to the cleave; healing has to be AOE to survive, and that generates big threat.