Will it be exhausting to do all 3 maths (Further maths, Math methods, Spesh Math) in one year? What will the workload be like?
In my opinion, yes. I would say that I am someone who really enjoys maths and 2 maths subjects in one year (methods 3/4 and further 3/4 in year 11) is a LOT. Sometimes I just get so sick of numbers that I go and do another subject. Not to forget, only 2 can count to your top 4 meaning if you theoretically get a 50 in all, one of them would only contribute 5 points to your aggregate.
In my opinion, I would definitely undertake methods, as it is a good foundation and fulfils many pre-reqs. Between further and spesh, it depends on what "type" of maths you are interested in. Spesh is a more challenging, complex extension of methods while further is more practical. You could always do spesh 1/2 and drop it for further 3/4, but not as easily the other way round.