Art is time consuming, and you might have to commit quite a few hours to Theatre Studies. No experience with theatre myself but i'm gathering there's a practical side, which will probably involve rehearsal hours. In terms of the theory, I don't know how much crossover there would be with Eng Lit (which might satisfy your needs enough). I'd drop one of those two (art/theatre), as to be honest, you can continue them as a hobby in your free time, either by continuing art or getting involved with an amateur dramatics group. Depends exactly on your motivations/what you want to achieve as to what's best for you. If you want to do well, your time is better focused on four than five, especially where creative subjects are involved.
Also, what are your GCSEs like? It's silly to put yourself under extra pressure by taking an additional subject you don't need if you're already going to face a lot of work ahead of you.
Best way to judge it would be to try it for a few weeks, and decide then which to drop. You should have an idea of what's expected to you, time/work-wise and whether or not you think you'll enjoy it. It might surprise you that you don't end up liking one of them as much as you thought you might.