I got A in GCSE and I'm doing further maths. One thing I'd say is that GCSE grades aren't usually indicative of A-level performance - I got the lowest GCSE grade in my further maths class (I think the other 4 all got A*) yet I'm probably on the highest UMS now. In FP1, which is the first exclusive further module, they got D, C, C, A and I got an A, for example. So you could be ok. I say could - there's not really any telling until you give it a go. Obviously, getting a U or a D or something at GCSE might be a different case. There is a big jump between GCSE and A-level - that goes for most (if not all) subjects, so provided you work smart, you might be able to do it.
You can take up to 4 applied modules in further maths - those can also be part of the ordinary maths A-level, so only the two or three (I think some boards have four?) further pure modules are exclusive to further maths. They introduce more areas of maths (complex numbers, polar coordinates, proof by induction, hyperbolic functions, matrices yah de yah etc etc) and explore other areas in greater depth (differential equations, summations of series, vectors). Have a look at your board's specification to see exactly what topics you'll be doing, maybe take a look at some of them to see if they look good to you.
Regarding the teacher not being good: if the teacher truly is rubbish, then get help elsewhere. Find another teacher to get help from. Ask on here. Try teaching yourself sections if that teacher's that horrendous.
I think 3 hours a day's a bit extreme. I'm teaching myself FP3 at the moment and I do less than that. But don't expect it to be easy, because it's not. If you do decide to take it, you can always drop it if you don't enjoy it/find it too hard. The fact that you enjoy maths is great, it means that you're more likely to do some independent studying, which will help your understanding.