its 25%.

Vishal barot explains it most intuitively and Guilherme most correctly:

The correct mathematical method is to calculate the probability (P) that its NOT heads (NH) on the third fli = Pnh. NH (=Tails) can only happen if the fair coin was used, which is 50% probability . (We've already deduced that i cannot be the Tails coin). The probability to flip a Tails on a fair coin is once more 50%. Hence Pnh = 1/2 * 1/2 = 1/4. Therefore the probability of throwing heads: Ph = 1 - Pnh = 1-1/4 = 3/4 or 75%.