Three - he bombed Hogwarts and hadn’t even known it. It was the first conflict that arose after Voldemort died. Ron was up in arms about Draco - insisting he bombed Hogwarts and Draco firmly refused, getting quite heated by the accusation. However, it wasn’t long until the memories flooded back to him due to Voldemort's death. Voldemort had imperiused him to blow up Hogwarts. It made him sick. He didn’t leave the manor for the first two months after that.