Just saying GCSE's are mainly about 2 things, structuring your answer and knowledge. Tbh GCSE's definitely are not impossible to get all 7s in, and 3 hours a day in my opinion is too much. Its better to start early with lets say 30 mins a day, and build up to longer stretches nearer the exams.
I'm in the year above you so didn't do exams tho, sorta basing of my sister's experience and others from years above. I did get good grades tho apart from some teachers being a *****
Just keep the determination (and for english start remembering quotes and poems early). (from personal experience) - Its easy to say that you will do 3 hours a day, but not reasonable, your mental health would probably be below the ground if you did this for 6 months. Just don't stress, revise for each mini exam, you are not a failure, even if you don't get all 7s. If you find maths hard, then just do loads of practice papers and remember the grade boundaries in the actual thing are quite low (60% or so for a 7 i think, maybe less). And also, if this is any comfort, because your year has had a bad experience with covid, the exam grade boundaries are probably going to be much lower than normal
Mr Bruff is really good for english
Free science lessons for science
i never really had too many problems for maths, but if you don't understand a topic, be prepared to put hard hours in for that
its good to start early, but don't overwork (never let revison take over social life, unless maybe on study leave or nearer exams), and always believe in yourself, there is no set time that guarantees a grade, but good revision with lots of practice papers is the best bet
good luck on everything