If you're applying for law you're better off taking one or two higher level subjects that are essay based (you've got that covered as you've picked 3!). Your standard level subject combination looks manageable sans chemistry. For the record, chemistry SL (new syllabus - exam in 2016 onwards) is quite a lot more difficult than the old syllabus. A lot of people I know are struggling with it despite everyone (people who took the old syllabus) telling them its easy - the old syllabus was but the new one which you'll be following is NOT as easy. Do take that into consideration! I seriously recommend you take biology in lieu of it. It's mostly memorization based and the concepts aren't all that complex (for the record, way way less complex than chem ones) and thus are easy to understand. However, if you dislike memorization/don't like the bio teacher/hate bio/prefer to do chemistry, stick w/chemistry!
History/Economics: Pick the one you're more interested in. I also recommend looking into the track record of the teachers that teach each course. I'm pretty sure one teacher is better than the other and/or gets better results. I'd recommend going for the teacher with the better record if you equally like both classes. I'll also add that economics is the easiest group 3 subject statistically speaking and history the hardest (2% usually get 7s in history, 13-14% get 7s in economics). In terms of usefulness, history is probably more useful for law because of the analytic skills you develop from the course that are useful for law.