So in June we join the college and if the pre placements start at September , so within 3 months we can get a job offer already ?

Yes you will end up having summer internship offer within 3 months of joining the college but this is summer internship and not final placement. Final placements happens in second year of mba unless you get a pre placement offer from the same.company you interned with

These are the Summer offers. So yes the summer placements are done in September and you have a job offer for your summer internship in 3 months.

Placements in September are summer placements which are not final placements. These summer placements are done for internship only which is part of curriculum of MBA syllabus. Every student must need to do internship for two months (April and May).