I'm not sure if you've checked these reviews yet: Math 1F03, 1M03, and I'm guessing you mean 1LS3?
I think the course you should take should depend on how much math you've taken. For e.g. you can't take 1F03 if you have Calculus and Vectors (12 U). So before you take 1LS3 or 1M03, you'd need the credit for 1F03.

1m03 shouldn't be too hard as long as you put work into it...