EDIT: just realized i missed your part about once every 15 seconds
so i deleted my overly massive post. that is more reasonable. once every other rocket punch basically.
still, 3 instants can be pretty massive and abused if not careful. its not game breaking, but it is still class defining. Tell me any merc in their right minds that wouldn't spec into that ability. once every 15 seconds my healer could lay out 3 5-6k heals (or bigger) over the space of the time that it takes a sorc to put up 3 bubbles. that's potent. or i can unleash 2 tracers and a fusion missile. and that's every 15 seconds. even a sorc has to wait 20 seconds after putting their bubble one someone before they can do it again. you would be able to dump close to 20k heals onto a tank in 3 GCD's.
Again, I like the idea. but even with 15s 3 is still pushing it. I could see one ever 15s. my gunslinger can crack her instant out once every 30 seconds when she uses her KB....us getting one every 15 seconds isn't a bad deal at all
you are correct that it would be class defining. there is a reason for it tho. and every Merc would want to spec into it for that very reason
bioware did a tremendously awful job designing the mechanics of Merc DPS. just piss poor bad. it is the root of all problems that Merc have in competitive PvP, and if real balance is ever going to happen, the basic mechanics need to change.
3 instants every 15s would not be overpowered. the amount of damage a tracer missile does wouldnt change b/c it was activated instantly. the main difference would be that Mercs would actually have a chance to fight back and defeat a melee opponent, rather than just hoping they can run away long enough for the melee player to lose interest.
you are still forgetting that a Merc would have to be in melee range to get that proc. yeah, 3 Rapid Scans in a row would output a lot of healing, but it would also require a lot of heat (3 in a row would put you at ~70 heat), and it also means youre in melee range of an enemy which as a healer you *never* want to do on purpose. the heat cost alone means this would not be something that you could do frequently; it would be good as a reactive mechanism for combating melee opponents; using it as an offensive tool is going to over-extend yourself and give melee opponents the advantage (they wont need to use a gap closer if you close the gap yourself)
Merc needs a significant buff. a side-grade, or shift of one strength to another strength, or whatever is just not going to cut it.
any change that would actually fix the problem is going to sound overpowered. merc is just that far behind