在三年级下册的英语中，以下几种情况的句子后面需要加ing：
现在进行时：
表示正在进行的动作或发生的情况。例如：
She is reading a book.
They are playing football in the park.
某些介词后面：
例如：
I am interested in studying Japanese.
She enjoys playing the piano.
作表语或动名词：
例如：
Her job is washing, cleaning, and taking care of the children.
Swimming is good for health.
固定句式：
例如：
likes doing sth.
minds doing sth.
finishes doing sth.
动词修饰名词做后置定语：
例如：
The running water in the river is clean.
The playing children are having fun.
希望这些信息对你有所帮助。
