What would I be missing out in the 1ls3 if I just took the math 1a03? Anything big? I'm still unsure of what I'll do 2nd year but I know I won't continue to life science year 2, if that influences anything. Maybe bio chem related, things of that sort. So would I need the math 1ls3? I currently have 1a03.

the courses are equivalent. you will not be at any particular disadvantage by not taking 1LS3. 1A03 will provide a better background in integral calculus than 1LS3 so if you a) like math a lot or b) may consider taking more advanced upper-year math classes then take 1A03. it is very possible to get a good grade in both, but 1LS3 definitely has the lighter workload.