how many hours does it take to learn these 3 a levels from scratch?Watch
For a person with average learning speed.
I am a teacher of Biology and Chemistry Alevel and i am happy to tutor you (at a slight cost) if that is something you'd be interested in
Usual timeframe is probably about 18 months. (September y12 to March-May y13) You could do it in less, depending on ability and support available etc.
If you want to start now with a view to sit your exams in 2022, that'll be a lot more time-pressured but could still be possible. I'd ask someone who knows your ability better.