[Urgent!] Need help choosing my 3rd A-Level!

I'm concentrating on 3 A-Levels for the next 2 years to ensure I get the best grades I can. I have chosen History and Geography and I am really struggling with picking the 3rd option. I don't have long until the deadline for changing subjects.

I'm not sure about what I want to do at university but I have some rough ideas:
-Law
-Geology
-Marketing
-International Relations
-Economics

For my GCSE's I got all 7/A's, however, I only just scrapped it for Maths

These are possible subject choices with my personal pros and cons:

Physics

Pros:
-Enjoyed it at GCSE
-Highly regarded
-Allows me to do Geology

Cons:
-Hard subject
-Spend more time practicing the maths
-Doesn't really link well with my other subjects
-Less likely to get a good grade?
-Doesn't open as much choices

Maths

Pros:
-Highly regarded
-Allows me to do Geology and Economics
-'Opens more doors'

Cons:
-Don't feel as confident at it
-I will have to spend a lot of time on it
-Less likely to get a good grade

Law

Pros:
-Links in well with my other subjects
-Essay based subjects come more naturally to me
-More likely to get a good grade

Cons:
-Limits my degree choices

If you can take it, I would choose Maths. It is a good A-Level to have, and as you said, it will help when you come to apply for uni. You said that you would be less likely to get a good grade, but you haven't actually tried it yet? If you are organised and work hard, you can do well!

I realise it isn't on your list, but have you considered Economics at A-level? It has numerous transferrable skills into Law, Marketing, International Relations and obviously Economics lol.

Yep, Economics was my first choice for my 3rd A-Level, however I can't do it at my college as the lesson times clash with Geography

That's a poor do by your college, but if that's the case, looking at your future prospects i'd be inclined to say go for maths or law unless you have your heart set on geology.
If you would be happy with any of the other careers mentioned, I would consider it to be a toss-up between which you would get a higher grade in between maths or law.

What do you think about Physics? It'll only exclude me from Economics and maybe universities that require more science subjects (I haven't found any yet)

I think physics would be regarded to have less relevance to the courses you listed. Also, try having a look at the A-level physics course, it's considerably different to the GCSE course. You may not necessarily enjoy the physics course at A-level like you did at GCSE.