This depends on what colleges you have chosen. If you haven't yet got any seat through counselling, then you have to go for third round. Or else, you can go for upgradation if the above colleges are better than the college you have got in second round of counselling. Usually in choice filling, we put top ranked colleges at top followed by average and low ranked colleges. Now after second round of counselling, if you have allotted seat in Suppose 'S' college and there are few colleges above S which are better than S, then you can surely go for third round of counselling. So even if there are no seats left in those colleges, you'll be stayed in the same college as second round. But if you haven't filled your choices like this then that would be risky enough as you have chances to get low ranked colleges.