This is a nice A level checklist that people might wanna keep in mind when choosing their A level choices. Keep in mind that this is in no way a completely exhaustive list. However my uncle has worked in university admissions for 20 plus years including a couple years at Cambridge. Anyone is free to add info or correct it if required.
Firstly the idea of facilitating subjects is more or less outdated even for Oxbridge. The list was published in 2011 and since then unis place much more emphasis on the requirements of the course.
But some things to keep in mind.
If you do not know at all what you wanna do at uni but want to do A levels then selecting atleast 2 from the following list might be helpful as they get accepted for a vast number of courses. The list includes:
Physics
Chemistry
Biology
Math and/or Further math
English Literature
History
Any Language (apart from english)
Other subjects that although not traditionally facilitating subjects are equally accepted and respected by unis: Geography, economics (Cambridge, along with most unis consider economics a facilitating subject if applying for economics) and perhaps english language (this one's a bit debatable)
Business Studies and economics SHOULD NOT be taken together and unis generally consider economics a more respectable subject than Business.
I would recommend against taking Accounting if it is one of your 3 A levels. It is not a traditionally accepted subject by most well ranked unis. (Edit: LSE will accept Accounting as one of the 3 A levels only if applying for Finance and Accounting)
RE, psychology and sociology are not useless A levels at all. Like I said it depends on the degree. They are on the preferred subject list for both LSE and UCL and I personally know a person doing HSPS at Cambridge now who had literature, history and sociology for A levels.
Media, for the most part should ideally be taken only if applying for theatre or film related degrees. Many unis, if applying for other courses not in any way related to media may not accept it as one of the 3 A levels.
Art can be a good A level. Obviously if applying for art or fashion for example. But I wouldn't recommend having art as one the 3 A levels and applying to Warwick for econ for example. However, art can be a good A level for architecture.
General Studies and Critical Thinking should not be taken as one/two of the 3 A levels.
Some course specific A level combination advice.
Medicine
Definitely have both biology and chemistry. I know there are some unis that will accept only biology and only chemistry but honestly, having both will open up your options much more.
If Cambridge is a potential medicine option, have math as your 3rd A level. Math usually isn't required by most other med unis. At the end of the day you need the grades. However, I would personally argue that biology, chemistry and math is one the best medicine A level combos you can take. So if you are good at it and like it then take math as your 3rd A level. But it is NOT a requirement (apart from maybe Cambridge). For example, biology, chemistry and psychology or biology, chemistry and history (for a bit of variety) or even biology, chemistry and physics can be very good combinations as well.
Law
There are no specific requirements but having a range of subjects can be helpful and make sure to have an essay subject. Good options include history, a language, literature, sociology, math and economics. A diverse range can be helpful. For example, math, history and literature or a language, economics and sociology or even a science.
Economics
Definitely take math. It's a requirement for most unis. If applying for LSE (straight econ) or Cambridge, definitely take further math if offered at your school. But if taken, you need 4 A levels. Taking FM as a 4th A level can be helpful for Warwick as well (straight econ) though they tend to be a bit more lenient with this. Take economics too if possible.
The absolute ideal combination for Cambridge would be math, FM, economics and history/literature. Oxford doesn't mind not have FM but they don't have straight econ. If you do not want to take FM and want to apply for LSE you could consider dual degree courses like Econ and history, econ and politics, PPE or econ and management.
Engineering
Definitely take physics and math. FM can be very helpful as well. A second science (ideally chemistry is good).
Architecture
Physics, math and art can be a very good combo (with FM if possible)
Nat Sci.
Try taking physics, chemistry math and further math.