Substituting foreign language class with the AP Exam?

Next year, I will be a junior taking all honors or AP classes. There are two slots left in my schedule that I need to fill, but there are three classes that I would like to take (AP Econ, AP Bio, and Honors Experimental Research Design, which is basically conducting research and designing an experiment that can be submitted into competitions, be published and whatnot…)

For me, AP Bio is a must. The other two are about equal, though ERD is tougher than AP Econ. Ideally, I’d like to take all three, so I was thinking that I could drop my honors French class since I’ve already taken two years of it. Instead, I would self-study for either the AP French exam or Subject Test to show that I continued on my own.

Is this a bad idea? How hard is it to study for the AP Exam if I am already getting A’s in my honors class, which is fairly easy for me? And if I keep French in my schedule, should I take Econ or ERD?