adding spanish or maths is NOT necessary, as unis only take into account 3 a levels, and generally are indifferent to people applying with four. It is better to get 3 A*s or 2A*s and an A than to get A*A*AA or A*AAA. you might want to do some research on each course you want to apply to - for law the russel group unis only need an essay subject such as history that you're already planning on taken, so you'll be fine!!