A level subjects

If you only like maths and want to make big money, I would highly recommend Maths, Further Maths and Computer Science. Maths and further maths are probably the most fun A-Levels to take, and also have the most weight as you will most likely end up in a high-paying job.

And don't get put off by computer science at GCSE, GCSE Comp Sci is really boring and meaningless, I heard that A-Level comp sci is much more interesting.

With this combo, you can go to university, study something like software engineering and enter a graduate scheme in one of the big daddy companies like google or facebook who will offer your big bucks.

Check this out https://www.levels.fyi/SE/Google/Facebook/Microsoft

I would echo what Further Maths Student said.

Maths and Further Maths is very interesting so I would definitely take them if I were you. As for the third A-Level, I would say pick whatever your second strongest subject is at GCSE. If you want to go somewhere like Imperial College for Computing, you need solid grades.

Universities that offer courses in software engineering and computer science appreciate that students can learn and most likely have learnt most of their coding knowledge outside of school. If you can communicate that in your personal statement to the university when its time to write applications for unis - you don't really need to do a level computer science. 

My friend got into a software engineering at university and he didn't even do computer science at GCSE or A-Level.

Here's what Imperial College say in regards to what they recommend for A-Level when applying for their computing course.

They also write on their website:

So doing computer science A-Level isn't something you should stress about, ask anyone that you might know who's done it at A-Level and see how their experience was.

Check https://www.imperial.ac.uk/computing/prospective-students/courses/ug/beng-meng-computing/
for further reading