✨ ベストアンサー ✨
以下のように考えます。
She had three sons, and all of them(代名詞・目的格) became doctors. 彼女には3人の息子さんがいたが、全員医者になった。
⇒ She had three sons, all of whom(関係代名詞・目的格) became doctors.
※them → whom + 関係代名詞は接続詞(例:and)の働きがある
whoを使う例文:She had three sons, who became doctors. ← She had three sons, and they became doctors.
参考にしてください。
You're welcome!😊
ありがとうございます!