Maths and the sciences are only very loosely related to Law, as they're quite formulaic. Law is too, to an extent, but draws heavily on well constructed arguments and coherency- skills which won't be learned properly with those options. Law as an A' Level itself also doesn't give you much of an advantage- in some cases it actually worsens your chances, or so I've heard. I didn't take it myself, but from what I've been told, very little of what you learn at A' Level is transferred to your undergraduate course.
Personally, I chose English Literature and History- these are staples, in my opinion. Both help you develop your argument building and get you used to reading a lot around a particular subject (more so with History). They're favourites to Law admission staff exactly for those reasons. Your third choice is less important, so you can pick what you like, but I'd recommend something which has a good bit of reading involved as well. I chose Geography- but that's far from the only option. You could do Law- as long as you do well in History and English, there's no reason why not to- the main cautions I've heard were against putting all your eggs in one subject (people doing unrelated subjects and Law, then wondering why they weren't made an offer). You could pick something more relaxing like Business, or something like Philosophy or Government and Politics, if those grab your interest.
Honestly, I'd strongly advise against pursuing maths-heavy subjects if you already feel like Law is the path for you. The subjects won't really help you in the long run, and might actually do you harm, as you'll have to acquire afforementioned skills both later, and at a faster rate, than most of your peers. If you're really drawn to sciences (as I was when I was picking my A' Levels), I suggest picking your favourite branch and sitting an AS in it. I did Physics- although I found the lack of a maths class alongside it to be more challenging than I'd expected. English and History alone will greatly improve your chances and relevant skillset.