LOCAL terms are involved.
If you have checked and concluded (correctly) that there is NO other Co-sharer than A,B,C then IT is OK.
Probably you have not seen any private unregistered agreement between A, B,C OR IT is not shown to you, or IT may not even exist….You need to confirm IT yourself.
Agreeing with Mr. ganesh rao, since IT being jointly held all joint holders should sign the sale deed.
The declinature of B, C to partition by boundaries points to some dispute/disagreement and boundaries are not decided.
Sale of undivided share is being offered to you by A. IT can be contentious issue infested with complications.
If you are properly informed and competent to handle such matters, check on your own and decide.
For any deal, preferably proceed further the expert advice of LOCAL counsel as already explained.
IT is otherwise…wise to stay away from such deals.
Rest is upto you.