✨ ベストアンサー ✨
(1) They [demmand]ed that their son (should)(省略可) come(原形) home for Christmas.
(2) He [suggest]ed that a tunnel (should)(省略可) be(原形) built instead of a bridge.
(3) It's [important] that we (should)(省略可) have(原形) self-confidence.
※命令・要求・提案を表す動詞、必要性や重要性を表す形容詞+that+S+(should)~(動詞の原形)
参考にしてください。