I've just finished college studying maths, further maths, computer science and physics.
If you're college permits you, I would just do the 4 subjects but push one to the side and focus on the main 3.
It's what I did and I ended up with 3 A*'s and a C in physics.
If you only want or can do 3 subjects then personally I would recommend doing further maths over physics as it is a very desirable a level to have and imo, easier. Plus, you will learn a very wide range of pure and applied mathematics far surpassing what you would have done at GCSE level. Usually small classes too, my maths/further class was only 10 people including myself.
I should also mention that if you have the intention of dropping one after trying them, you may have to wait longer depending on how you're college runs the course, my college taught the normal maths modules twice as fast (twice as many lessons) before teaching the further maths modules so you may get an idea of the pace you would be working at but you wouldn't get an idea of what the further maths is actually like for a couple months probably.
Sorry that my reply is kinda disjointed, I was just writing things down as I thought of them 😂
Feel free to ask me about the sort of stuff you would be doing on the course, of course it will be a bit different than what I did but from what I gathered from my friends in the year below me, the content hasn't changed a great deal.