You have a Holy Priest, I would have a dedicated keys for that priest.
Since I 3-box using Holy Pally as healer, I used U, I, and O Keys for individual keys.

Key U: /cast [target=Party] Holy Light
Key I: /cast [target=Player] Holy Light
Key O: /cast [target=Party2] Holy Light

Party1 in my Holy Paladin is always the group leader, which is my Main Tank.
Player heals himself
Party2 is the my DPS.

You can replace those with specific names also.
1.Sanctume (tank)
2.Semihrage (holy paladin)
3.Healmon (shadow priest)

Key U: /cast [target=Sanctume] Holy Light
Key I: /cast [target=Semihrage] Holy Light
Key O: /cast [target=Healmon] Holy Light