If I understand paragraph 1 of the statutes of the Nobel Foundation [1] correctly, a settlement between the Foundation and the heirs of Nobel forbid it from deviating from some "main principles", which include that the award shall not be divided into more than three parts. So presumably, if they amended this rule, they could be sued by the heirs. Also, paragraph 23 suggests that any amendments to the statutes must respect paragraph 1.
I think the fund for it was set up from the trust of Alfred Nobel in accordance with his will. So presumably whatever law in Norway involves trusts and enforcing peoples last will would be binding on the fund, and thus on the committee
[1] https://www.nobelprize.org/about/statutes-of-the-nobel-found...