A contract to make a will or devise, to revoke or not to revoke a will or devise, or to die intestate, if executed after January 1, 1981, can be established only by
(1) Provisions of a will stating material provisions of the contract;
(2) An express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or
(3) A writing signed by the decedent evidencing the contract.