I've always been under the impression that, for magic healing, damage stacks and isn't kept in separate groupings.
For example this sequence of events is legal: Take 3 boxes of damage, healed for 3, take another 3 boxes of damage, healed for 3.
But for this sequence you'd have to wait for natural healing to take care of the rest: Take 3 boxes of damage, take 3 boxes of damage, healed for 3.