Law is the sort of subject for which universities often aren't explicitly bothered which subjects you've taken. Recommendable is History (which you have) and another essay subject (often English Literature, as it's the most academic, but I can't see Sociology being a problem here). On very few courses are there requirements for three A-Levels to be in a specific subject, but on most courses there are requirements for three A-Levels to have a certain achievement level (AAA, ABB, etc.).
Do you enjoy Art? Would you want to continue it for the rest of Sixth Form? It's not a 'facilitating' subject, but it's recognised as being a very difficult one (particularly in terms of time management with coursework), so I cannot see having it holding you back, provided you get a good grade.
Don't drop anything you don't want to drop - you'll only live to regret it!