capital gain ?

Hi, i had purchased owner ship flat during 1984.
Society went for redevelopment, agreement between flat owner and developer signed as Alternate permanent accomodation during June 2015.
Possession of flat received during July 2018,
Now I want to sell flat to buy another flat immediately.
A) Is this considered as a short term Capital gain tax ?
B) If yes then to what extent it will be short term Capital gain?
C) Can I get waiver of short term Capital gain if full proceeding is reinvested in new flat or this clause is limited to long term Capital gain?
Kindly respond if anyone has come across such scenarios or are aware about the implications. Thank you