Nice idea... I see 2 paths that this weapon may take:
1- Being able to heal you, thus having to keep the cost at 3
(won't be THAT OP, but it may need some nerf, like 3/4 atk).
2- Only healing creatures, which the game might make use someday, and that would be an interesting mechanic, but that way maybe it could cost
or even switch its power name to Regeneration and make it cost
(my favourite idea).
I say change the cost to 1
for the ability (call it Regeneration), and make it as to be unable to heal you, or that it dissappear once you got poison, no matter if you had 100 Regen counters (like the way poison/purify work).
As I said, nice idea.