[argonaut]

Before you do what soneca said, consult a reputable, good lawyer with experience in IP/cofounder issues (not a cheap random lawyer). Tell your lawyer everything, your whole story. Don't leave out any detail. Tell your lawyer what you plan to say to cofounder C (i.e. the dialogue soneca suggests you should have), but before having that dialogue ask the lawyer if you should go ahead, and ask the lawyer if there's anything specific you should say, and anything specific you should avoid saying. Do not talk to cofounder C without consulting a lawyer first. You don't want your words to be used against you in the future.

I am not a lawyer, which is why it is imperative that you do consult a lawyer before proceeding in any way.

Right now, since you aren't successful, you don't seem to have anything to lose. But if you guys do gain traction, this problem could come back to bite you if left unaddressed.