I use key clone for my FTL set up, but I would think HKN would have the ability to do key mappings like KeyClone does.
That being said, I would set up two buttons on each druid
Macro1 (bound to key 1)
/cast barkskin

Macro2 (bound to key 2)
/cast [mod:CTRL,nomod:ALT,nomod: SHIFT,target=ToonA][nomod:CTRL,mod:ALT,nomod: SHIFT,target=ToonB][nomod:CTRL,nomod:ALT,mod: SHIFT,target=ToonC]
Healing Touch

Then would make the following key map

ToonA
1 -> Barkskin
Alt+2 -> Barkskin
Shift+2 -> Barkskin


ToonB
1 -> Barkskin
CTRL+2 -> Barkskin
SHIFT+2 -> Barkskin


ToonC
1 -> Barkskin
CTRL+2 -> Barkskin
ALT+2 -> Barkskin


This way, which ever your "leader" character is would barkskin, the other two would heal it.