What is the ruling if the Imam makes three prostrations during the prayer?

If the Imam adds an extra prostration intentionally, the prayer becomes void. If it is done out of forgetfulness, the prayer is not invalidated but requires the prostration of forgetfulness (sujood al-sahw). If the follower (ma'mum) knows that the prostration is an addition, he should not perform it. If he performs it intentionally and knowingly, his prayer becomes void.