If you want to reflect it's heads why not just say it can't be blocked by less than 3 creatures?
I wanted to come up with a new way of describing the damage that a three headed creature would deal. And classically, Bellerophon did block the Chimera so to speak.
You can't just make up last strike, you kind of have to work within the rules.
That mana cost is even worse. And what makes this have to have 3 heads? And are you aware that Magic already has a cycle of Chimeras (artifact creatures from Visions)?
1. I didn't know I was outside of the rules in making up a mechanic like this one.
2. Chimeras have three heads
3. I am aware of the Chimera cycle in Visions, but I was really dissapointed by it. It seemed as though the individual chimeras were something closer to Voltron than the Chimeras of classical and medieval lore.