i didnt take any sciences, but all three maths. I think it all comes down to time management and wisely dropping/adding courses that you can manage per semseter.
If your not the smartest cookie and if you think you cant balance your courses, take couple of courses in night/summer/pvt school. Get those courses finished FAST.

Yeah, bcd92 said it right. Time management is everything. Like tripleabatt, I had all 3 sciences in one semester, and it was tough, but doable. You just need balanced semesters - I don't like having biology and chemistry in the same semester because they generally have the highest workloads of any course. And you need to time manage, don't study 2 days before the test when you have 2 or 3 tests in the same week. If you review everyday, don't leave stuff last minute, it is definitely doable.

Lock yourself in your room and study all day!

No, but seriously, you should prioritize, and effectively manage your time. Also always try to get a enough sleep (at least 8 hours) NEVER pull allnighters! If you prioritize and manage your time properly you never have to.

And if you want to do super well here is my advice: Identify the waste in your life and cut it out! For me, I gave up video games and it is the best choice I've made, honestly I can not have imagined how much I have been able to achieve this year when I was in Gr 11.

Definitely prioritize your time, if you are having difficulty with one course work on it for a bit longer. If you are going to take 3 sciences and 2 maths take good notes&review them daily. For your elective try to choose something less challenging so your course load isn't so bad. My grade 11 year looks like this:
Eng 3U-English
FSF 3U-french
SBI 3U-biology
SCH 3U-chemistry
PAF 30-Girl's fitness
MDM 4U-Data management
PSE 4U-Exercise science
plus intro to anthro, psycho and socio (I forget the course code for that but it's mixed)

as you can see girl's fitness lightens up my schedule. So first I asked myself "hmm what course is going to be the toughest?" and that would be exercise science so that is the course I focus on first and for most, then bio, then chem, then French, then data management then English and then psychology. So what I do is review my notes everyday in that order for each class (if I have any) or whatever was taught in class that day. This way by the time I start to wear out it's a class I have a better understanding of:) I took thorough notes and paid my full attention to the teacher when speaking. For math I did lots of extra questions in class but didn't review as much. So just really managa your time around the difficult courses then your easier ones.