Honestly, you would be far too busy in your first year to try and take on 3 full A levels in addition to the amount of studying, extra reading, lectures, sports, society activities, extra curriculars, boozing, recovering from said boozing, etc. that you’re likely to be doing as a fresher. I tried to take on one extra 20 credit module (which I think is not even equal in work to A level) in my first year and couldn’t handle the extra workload and barely a month. If you’re really keen on getting the 3 A levels then think about doing one of them per year? That might be more manageable. Alternatively, I’d highly recommend getting yourself a summer placement or even an “industry year”. This would immediately put you above even straight A’s A level students. Once you’ve got your foot in the door sort of thing. Like you, I took a different oath to uni without A levels and did a science foundation year to get into an RG uni. Since graduation give gotten a job with one of the best engineering consultancies in the country, all without an A level in sight. Having said that, I have no idea what the application process is like for law firms. They obviously do whatever they can to whittle down the number of applicants. There are only ever a few places for every 100+ applicants nowadays, in all sectors and industries though not just law.
Hope this helps!