第一种情况:fit in (BELONG) :phrasal verb
1.to feel that you belong to a particular group and are accepted by them:
E.G:It's no surprise she's leaving - she never really fitted in.
2.If one thing fits in with another thing,they look pleasant together or are suitable for each other:
E.G:It's a very nice sofa but it doesn't fit in with the rest of the room.
第二种情况:fit sb/sth in (FIND TIME):phrasal verb [M]
to find time to do or deal with something or someone:
E.G:Dr Jones is very busy but I'm sure she'll be able to fit you in tomorrow.
第三种情况:fit in with sth:phrasal verb
If one activity or event fits in with another,they exist or happen together in a way that is convenient.