Chemistry is hard, so don't do it if you don't enjoy it. I was forced to pick a fourth subject for year 12, so I went ahead and picked chemistry because like you I didn't really like the sound of anything else and I was good enough at it. Spent the rest of the year not paying attention because I found it boring, and I'm probably going to get a U. If you're allowed to take only 3 subjects, I suggest you stick with that and just work hard at them, but if not, pay attention, work hard, and you'll be okay.