Tax liability on sale of flat

I had purchased a 2 BHK flat in 2009 and recently booked a new 3 BHK Flat recently which is under construction.The cost of the 3 BHK flat is much high so i will have to sell ealier purchased 2 BHK flat during the time of possesion of 3 BHK. if i sell this 2 BHK flat and invest in 3 BHK before possession. will i get the Tax benefit? i.e. No long term capital gain tax from 2 BHK

or It is mandatory to sell the first flat first then book the new flat to get tax benefit.

kindly put some light on that