Which course would be more beneficial to study in university and make me successful as possible? 1- History or Politics 2-Medicine 3-Law
Which course... at Cambridge? (You are asking this in the Cambridge forum.)
The most beneficial course is the one you will most enjoy studying for 3+ years.
But do you have A-levels (or equivalent) that enable you to realistically study all 3 of the courses you have listed? To read Medicine at Cambridge you really need 3 sciences including Chemistry.