Hi, first of all I'd just like to say you're not alone!
Like yourself I'm also doing three sciences and maths. I started revising 'properly' 2 weeks ago however have already seen what I'd like to think is a solids grades worth of progress in each subject over this period. This was with 5 hours a day average over the last 2 weeks (had 3 days a week where I would do very little revison and and go out for a change of scenery and the other days were 6-8 hours of work. Bearing in mind that this was during the holidays.) I'm quite sure that in over a month we can get 4 As as long as we revise smart and stay focused on the goal at hand. As for doing 100 hours per subject, thats on average 10 hours a day, does this include around 5 hours of in school learnign each school day? But yes, I believe it will be more than long enough as long as you revise smart!