必定：something that will be surely happened, 例句：他必定迟到 (he will surely be late) 必要：necessary, need to , 例句：我们有必要带瓶水。(it's necessary to take a bottle of water) 必须：must 例句：你必须听我的 （You must listen to me)

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。