Dear Querist, in relation to your query my opinion is as follows:-
1. Previously, DDA or any other private builder used to sell floors and roof rights separately but these days after many judgement of Hon'ble Supreme court it is very well stated that the roof rights can't be sell separately but with the floor only.
2. While doing agreement please mention that you are buying third floor along with the roof rights.
3. In case you don't mention "Third floor+ roof rights" other floor owners can put their claim on it. So, it is advisable to mention it specifically.
4. There are many judgements which make the owner of second floor absolute owner of roof rights as well.
For any further query, feel free to call.