Whoever said you should study for four modules is just wrong I think. Studying for four units means you either have to invest way more time, or you just won't be as good at all four as you would be at three.
I am doing, Linear, Matrices and Business and I find none of them challenging, linear is good because it has some similarities with core and is also pretty easy (apart from Linear Programming). Matrices and business are both very straightforward and just come down to making sure you pick up on the tricks.
I would suggest just doing whatever three modules your school does, your SACs are great practice and don't underestimate the teacher's ability. They will help you more than anyone in understanding every nook and cranny of your module and one you learn yourself won't come close to one taught.