I use the exact same setup with Pally, 3 mages and Holy Priest. The way I do it is with Character specific healing macros using a Logitech G11 Keyboard in conjunction with the Bindpad mod allowing me to have a little block of 6 keys grouped together one for each of my toons and one for group heals like prayer of healing. The 2 Tank heals are also on the main bar for the priest.

Each of these uses Ctrl and Alt as modifiers which are really easy to use with the pair of extra buttons attached to my mouse...

Little Tank Heal Macro
Flash Heals normally, alt press for power word shield on the tank. I have a separate macro for renew tied to the same button as holy shield, but that one's hardly rocket science...
/stopmacro [mod:shift]
/tar [target=focus,help];party1
/stopcasting [mod:alt]
/cast [target=party1,nocombat,dead] Resurrection
/cast [mod:ctrl/alt] Power Word: Shield
/cast [target=focus,help][target=party1] Flash Heal

Main Tank Heal Macro
Bread and butter Healing Macro alt press activates trinkets and casts circle of healing on the priest instead for quick heals on the party.
#showtooltip
/stopmacro [mod:shift]
/tar [target=focus,help];party1
/stopcasting [mod:alt/ctrl]
/cast [mod:alt/ctrl, target=player] Circle of Healing
/use [mod:alt/ctrl] 13
/use [mod:alt/ctrl] 14
/cast [target=focus,help] [target=party1] Greater Heal


DPS Heals
Healing macro for DPS toons, does a cast sequence of power word shield, renew and flash heals normally, straight flash heals on ctrl press and greater heal on alt press
/stopmacro [mod:shift]
/tar [target=Neeva,help][target=Darnoc,help];Party3
/cast [nocombat,dead] Resurrection; [mod:ctrl] Flash Heal; [mod:alt] Greater Heal
/castsequence reset=10 Power Word: Shield,Renew,Flash Heal,Flash Heal,Flash HealThe big heal and little heal are tied to 2 buttons on the main bar that no other toon uses since I'm spamming them all the time. I also have straight Prayer of Healing bound so I can do that when needed. Most of the party heals come down either Circle or Prayer of healing for my group, but I do frequently use the character specific....

Obviously this setup is somewhat complicated, but it works really well for my purposes...