My dad brought 3 flats in 1978 but made single agreement

Shares certificate is itself legal documents for bank loans etc. While your sale deed is the proof that the property has been legally transferred in your name, a share certificate is a legal proof that you are the rightful owner of the co-operative housing society's shares. It is the duty of the managing committee to issue share certificates after due diligence.

In response to your query,

A cooperative housing society issues a certificate as verification that a particular member is a registered owner of the cooperative housing society's shares. A share certificate must include a distinctive number, together with the name of the housing society member, the number of shares granted to the individual, and the amount paid thereon, according to the state's model bye-laws.

A sale deed serves as the primary legal document confirming the sale and transfer of property ownership from the seller to the buyer. The sale deed is the primary document through which a seller transfers his right to the property to the buyer, who subsequently becomes the owner of the property in its entirety.

Now as you mentioned that both these documents are in your possession therefore I don't thunk you must have any troubles in making the sale.

Regards,

Aryan Raj 

I did not know the answer to the question. Hence I wanted to know what advise experts give. I find the question is not adequately answered. There are 3 share certificates but only one sale deed. If he wants to sell the 3 flats to 3 different person and they finance the purchase through 3 different banks, what all the documents are to be given to each bank for custody. As there is only one purchase deed how can it be split into 3?