1. insist for a supplementary agreement in which the builder and owners agree on their respective flats in the new building
2. why 2 agreements? just one tri party agreement is sufficient
3. under this tri party agreement, the land owner will agree that the flat which was allotted to him can be sold by the builder to you. So no power of attorney will be needed as the owner himself will become a party in the tri party agreement
4. also if above tri party agreement is executed then even the flat distribution agreement may not be required. But since you are saying that there are 3 owners, its better that there is a separate supplementary agreement which will describe the flats of each party i.e. the builder and 3 owners
5. another option can be that in the tri party agreement, the other 2 owners can become confirming parties. so the supplementary agreement can be dispensed with, that is, wont be required, since all owners will be agreeing in the tri party agreement itself