The results will be released in three months. "after three months" is meaningless, because it could be anything from 3 months to eternity.

Yes you can but the meaning is slightly different. 'In 3 months' means 3 months from now. 'After 3 months' means it will take more than 3 months. You can also say 'within 3 months' which means before 3 months have passed.

I think so. In three month means it will cost at most 3 months and After three months mean it will cost at least 3 months.