Problem: Having three seperate rollers is broken, from gameplay, UI, and code perspectives.
Solution: Switch to point-based with an optional "randomise" function.

Plan:

The idea is to use the point-based system all the time. A faux-autoroller is used that simply prioritises different stats and assigns points semi-randomly within that system. Make sure there are decent instructions, too.

It allows you to balance the game so that point-based is the default. You could make some carefully selected internal distributions for "roll my own" and "set variance", so paladin starts with 17/0/?/14/14, etc.