you really don't need to be picking your alevel options in year 10, but it's good that you've got a rough idea of what you want to do!
as someone who's also doing STEM (granted the T, not S) I would suggest doing Maths and two of the others if you're only going to do 3 subjects as it means that you have some transferable skills between all the science. Which of the two sciences do you like the most? or what kind of science do you want to do in the future? cause I would base your options off of that but if you aren't sure / like all the science equally then maths, biology and chemistry is probably the best option as maths is most similar to physics in terms of content from what I've heard.