But doesn't AQA have a FP4?
It really depends on you as a person - what sort of modules are you prefer/find easier? I'm doing D2, as well as FP3 and S3, and sure it's easy to learn, I'm on Chapter 4 of 5 already after a couple of weeks, but it's not as easy to do well in the exam as you have to include every single step of the algorithm you're doing - I'm not like that, and will often do steps in my head without writing them down and find while I've got the right answer, because I didn't write down exactly what I was doing, I didn't get full marks.
If you're very methodical, then D2 should be very easy for you, otherwise I'd say stick to S3 or FP3 as both are good modules, although they will be harder to self-teach.
FP3 for me would be easier than S3 definately...