Are you using array or point buy?
With the standard array, the most a fighter is going to get in their 3 main stats is 17, 16 and 14 (I think). Getting all three to 20 is going to require all their ASIs. If they want another stat raised, for proficiency reasons, then they will have to trade off.
Problem is (with the exception of the EK), there is no "3 main stats". They have their attack stat, and Con.