I hear my parents ___ back in three days. 填come还是will come?为什么

will come 其实这句话是一个宾语从句， I hear (that )my parents will come back in three days. (听说...)
不是你说的 hear sb do sth 结构。(听见某人在做某事)
试比较：
1.I heard （that） they lost the match yesterday.
2.I often hear the girl play the violin.

will come 因为是将来时嘛

will come
表将来

I hear my parents _will come__ back in three days.
in three days与将来时态连用

will come
这个的完整句是 I hear that my parents will come back in three days.
这里把that省略了，hear 理解为听说

如果hear为听到...的意思，用hear + do\doing的结构
例如，I heard him screaming. 我听到他在尖叫。

用：COMING，这里现在进行时表将来。也符合COME 的用法。