Joint Property Purchase

We have purchased 2/3rd of share in an agricultural land from person A & B and remaining 1/3rd of share was purchased by another person from C. Both the transaction were executed seperately and not connected to each other. I understand that this entire land was earlier a property of 1 person and he sold it as shares without defining the boundary for each share. Now another person is not allowing us to do agricultural activity in the 2/3rd of share purchased by us because he wants to exchange his 1/3rd share which has less water with our share which has more water. He is saying that as there are no boundaries given separately for your 2/3rd share, and his 1/3rd Share, he can take any 1/3 share no matter wherever it is. And as it is deemed to be a joint property because of no separate boundaries for each share, being a joint owner, he is not allowing us to do agriculture.. please provide your insights and help us with any case law or validity of his argument.