I think you'll be fine, speaking as a medical applicant of course, but one thing I think you should be aware is that even one essay subject is a lot of work. I'm taking history and it does cut into a lot of time, and I do find I spend a lot more time with history than I do with my other subjects (Bio,chem, and maths) but that may be just me. In terms of universities, they don't mind either way (as long as you have bio and chem :P)
Good luck!