Hi Vihaa Harish.

Normally what you have to do is while accepting your seat that you got in round 2, you have to choose for upgradation in coming rounds. This procedure can different for different colleges or states. FOr JoSAA, you have to choose the float option. And in some cases, you have to do that while admission. PLease go through the guidelines of the admission procedure.

Good Luck.

You can simply choose the float option in second and you automatically become eligible for third round counseling but keep in mind that by doing so you'll loose your seat that you got in second counseling.
Hope it helps

Hello
It would we better if you could tell us the entrance exam details, branch and Colleges which you are interested to apply for. You can apply for the third counseling in a similar manner as you did for round 2 , if you get a better college than your previously alloted  one , your seat will be updated to the new college else the previously alloted seat is retained.