I wouldn't recommend science at a-level if you're working at a 5-6 in combined, don't get mislead by your teacher predictions as they will be inflated. the jump between GCSE science to a level is HUGE. I got a 9-9 in combined, take Chem and Bio and have been averaging A/B's but this is with 3-4 weeks prior revision for topic tests. My friends who took a level sciences with a low combined grade are struggling big time, they find it harder to understand concepts because their foundation knowledge isn't strong enough, even with a lot of hard-work they will automatically be behind and sciences only will look good if you do well in them (B-A grade). If you want to take 3 essay subjects be prepared for a huge workload, you need to be able to be okay with maintaining that workload too. I wouldn't recommend law as an a level either not going to lie, it isn't that applicable. Taking 4 a levels is a waste of time, if you want too tho you should take one subject as an AS and call it a day.