And would I drop a bottle or replace it with cows milk? 
From 1 you can move to cow's milk, either by just swapping it straight or reducing the formula and replacing with cow's milk bit by bit. It can make some toddlers a bit constipated at first while they adjust so it's something to be aware of.
It's great that he is using straw cups not bottles so you don't really need to worry about it. If he is eating well with the 3 cups of milk you can keep them if you want, but if he's not eating well I would drop one or reduce them all a bit as milk can be really filling. They only need 12-14oz of dairy to get their needs met from 1 and that includes food so he's getting plenty!
I'll move this over to eating for toddlers too!
Laura