What should I do about school?

I'll be entering my senior year this fall. I only have two lab sciences, but I need three to apply to most of the colleges I want. I was thinking about taking an online course during the fall semester. Here are the benefits as I see them:

1. I'll have guaranteed that I'll get the lab science that I'll be promising all the colleges I apply to (I'm afraid if I wait to do it in the spring, something'll come up and I won't be able to take the course and colleges will rescind whatever acceptance offers they would have given me at that point).

But the drawbacks are my schedule for the fall. I'll be taking three AP courses (max. four per semester at my school), and one of them (English) is said to be by far the toughest on campus. I don't want to let my school grades suffer nor do I want to go crazy with all the APs and college applications at that time.

But, again, I keep thinking how hard can an online geology class be? (<--- This is what I really need answered. Will it be a lot of work? If it'd be like half an hour of studying a day, I'm pretty sure I can manage that for a semester. But if it's going to be a good amount to deal with, as in I'd have to work to get an A, I'd rather find a way to put it off until spring, if possible.)

I don't know. Should I just accept that my fall semester is going to be miserable? Or do you think the online geology course at a community college will be easy enough to handle, even if I'm assured that all my other courses at school will be killing me?