Advance tax upto sept 15 to be paid is upto 45% of total tax liability (total tax less TDS) so you don't need to pay full tax on 3L LTCG before sept 15.
Subtract 1L from your total 24-25 LTCG and calculate 10% (not assuming the budget changes right now) and pay 45% of that 10%. So say 3L - 1L exempt so 2L*10% is 20k and 45% so okay ok by sept15