"That man washed his hair three months ago."
Translation:그 남자는 세 달 전에 머리를 감았어요.
Quick fact: The Chinese character (Hanja) for 삼 is actually called 석 三 (삼).
When Koreans say "wash one's hair", we use the verb "감다".
머리를 감다 (현재형) 머리를 감았다 (과거형)
e.g. "남자가 머리를 씻었다"? is very odd. We never use this sentence. It sounds like the man has no hair.