A-Level Choices, History or Chemistry?

Just take FM as your third and be done with things.
If you want to take a 4th just pick whatever you prefer.

can fm be taken as a third? multiple people have told me that fm can only be done as a 4th, unless my school is just weird.

There's no reason you can't take FM as a third subject in principle. Of course schools may only offer it as a 4th A-level though, and what you may be seeing commented on by others is that some degrees (such as medicine) may only accept it as a 4th subject. For going into physics though it's perfectly acceptable to be doing physics/maths/fm (it's probably one of the more if not most common combinations I would expect).
The only potential caveat to be aware of will depend on how your school teaches FM - some schools arrange their maths/FM teaching such that they cover the whole of A-level Maths in year 12, and students take the exams for it at the end of year 12, then the whole of A-level FM in year 13, and then students take the FM exams in year 13. As some unis (such as Oxbridge) prefer to see applicants have completed 3 A-levels in a single exam period, you may need to plan to do a 4th subject anyway in that case, so that you're doing physics/FM/3rd subject in year 13 (after doing A-level Maths in year 12).
Other schools just teach them in parallel or otherwise interweave the teaching of the two and you take all the exams for both maths and FM at the end of year 13 (along with your other subjects i.e. physics in this case) as per usual.

I was in the same position as you last year and chose chemistry alongside maths physics and chemistry. If you want to do physics at Cambridge, check the course online- it is called natural sciences and they almost require chemistry a level for it because in your first year you must choose three subjects out of- chemistry, physics, biology, evolution, physiology, material science and earth science. Chemistry goes really nicely with physics and maths too. I know people who do history with sciences and either want to give up because they don’t see the point in it, or struggle because the subjects don’t match well. Good luck with year 12!

They do in principle accept people applying indicating they intend to continue with physics who have done physics/maths/FM. OP could then take materials science and earth science or possibly one of the bio options. However OP probably might be less competitive certainly.
Granted equally they could apply to maths for the maths with physics option then swap into natsci physics in second year which is a predefined pathway available to maths with physics first years on the maths course (as from second year onwards they could and would presumably just be doing physics and maths options).
There is of course also Oxford as an option instead.

This is true, but even then materials science recommends having chemistry a level, and OP may decide that they want the entire freedom of the natsci course (which is its defining feature). Either way, both of us agree that further maths or chemistry is a better option than history, only that I reckon further maths as a fourth is best, you can always drop it if the work is too much.

So far, I've chosen Physics and Maths. I'm considering Further maths, as my dream career is Theoretical Physics and to study in universities such as Oxford and Cambridge. I've narrowed it down to Chemistry and History as they're both academic subjects and, as far as I'm aware, are both quite respected by universities. I'm not especially interested in either, they both rank about the same in my books. But I want to choose one which will most help in my future, and I've no clue which to choose. I'm leaning toward Chemistry, because obviously Physics is a science and History isn't. But I got a 66 in combined science. Due to my circumstances, I understand this is quite good as I found my interest in Physics during the middle of year 11, 6 months before GCSE's. Obviously it was too late to do triple science, and I was sat at a predicted grade 3. After discussing with my teacher, she gave me a shot and I got the 66 (higher tier). Exact same with maths. So my chemistry skills aren't amazing, but I'll do whatever amount of revision and learning it takes for an A. So, if you were in my situation, what A-Level subject would you take? It doesn't even have to be one of these.

How did you do in history and other essay subjects?

History I got an 8, English Lit a 6 and English Language a 7. If you include Computer Science (I don't personally) I got a 4 due to my horrendous coding skills, but my 'essay' skills were almost top of my class.

So far, I've chosen Physics and Maths. I'm considering Further maths, as my dream career is Theoretical Physics and to study in universities such as Oxford and Cambridge. I've narrowed it down to Chemistry and History as they're both academic subjects and, as far as I'm aware, are both quite respected by universities. I'm not especially interested in either, they both rank about the same in my books. But I want to choose one which will most help in my future, and I've no clue which to choose. I'm leaning toward Chemistry, because obviously Physics is a science and History isn't. But I got a 66 in combined science. Due to my circumstances, I understand this is quite good as I found my interest in Physics during the middle of year 11, 6 months before GCSE's. Obviously it was too late to do triple science, and I was sat at a predicted grade 3. After discussing with my teacher, she gave me a shot and I got the 66 (higher tier). Exact same with maths. So my chemistry skills aren't amazing, but I'll do whatever amount of revision and learning it takes for an A. So, if you were in my situation, what A-Level subject would you take? It doesn't even have to be one of these.

if i were in your place i would take chemistry unless you think that taking 2 science classes will be too difficult or draining

If two sciences are too draining, OP might not be fit for theoretical physics.

If two sciences are too draining, OP might not be fit for theoretical physics.

Being perfectly honest, it's not that they'd be draining, it's just that chemistry isn't something that massively interests me. Therefore, I'd just find it harder to understand. Whereas with maths and physics, sure they're hard but I am deeply interested in and love them so I'm more willing to work to my fullest extent.