first of all
必须、必定 adv.
必要 n.
有必要 adv
so ,next 必定、有必要、必须的差别:
e.g.: two football teams, Team A and Team B:
A必定赢B。(A is better than B, I believe , A will win.)
A有必要赢B。(A have to win B, maybe win,maybe not ,i‘m not sure, but indead A need 3 points.)
A必须赢B。 (A have to win B, right, I have bet "A win" for 300 dollars, please , A have to win , I bless.)
So ,they can't insead of each other.
不知道解释清楚了没有,希望能对你有所帮助。