1. There is no provision of law which stipulates that possession of flats located at the same tower shall have to be given on the same day.
2. The matter can be dealt with issue/case wise i.e. if all the flats in the tower are not ready in finished condition, then the builder can argue that he will give possession only after finishing the flats which will take the required time.
3. If the the brochure/agreement for sale stipulates that the possession of the flats will be handed over only after completion of all the flat of the tower then only you can claim that possession of your flat also should be handed over to you at the same time along with the other f;lat owners after collecting the agreed consideration from you.
4. Actually the builder is the right person to state why is he not handing over possession of the flat if it has already been finished.
5. From the query it appears that the builder is taking 3 years time to hand over the flats after taking the bookings for which no charge of fraud can be brought against him.