Good luck, I got 5 9s including all 3 sciences at GCSE this year and it’s completely achievable (not without work)!
Most schools in my area said that anyone getting 5 or below did combined foundation, and only people getting 7s or above did triple higher (what you’re wanting to do). Point being that a lot of schools will only let the best students do separate higher tier science because of how high the grade boundaries can be, so you need to be in the top end of your year group. Just work hard for mocks, and when you’ve done better than most of your year go to your head of science and ask to do separate higher!