My view is that a Law applicant should maybe have at most two humanities subjects, but avoiding the likes of sociology. I am firm believer that I good lawyer also has the analytical skills fostered by the numerate A Levels such as Maths, Physics etc. So I would advice you make your third subject one of these. To illustrate this point, a number of physics graduates each year go on to train as solicitors and barristers each year, according to the Institute of Physics. They obviously stress how attractive something like physics is to these professions.
Whereas someone who has done all humanities A Levels does not ever really stand a chance of getting into any job/sector that has some dependancy on analytical skills etc.