Ltcg / uc flat / selling before or after possession

1) If I sell before possession , will the gain be LTCG ?;

Before getting possession, what you can sell is only the right to get the allotment. WHETHER THIS RIGHT YOU HELD FOR MORE than or LESS THAN SIX MONTHS,  will have to be ascertained from the relevant documents you executed with the Builder.

2)  If I sell immediately after possession ( means after registry in my name ) ,  will the gain still be LTCG or I have to wait another 3 year after possession to avail LTCG ?

You become the owner of the apartment only on registration & your getting the possession. Therefore, for the capital gains to be long term, you have to hold the property for 3 years after the registration, unless you had paid the full price & had been put in possession earlier.