hmm...it's hard to say because i took 1/2 in high school and learned 3 on my own, but i'd say that:
-3 is probably the easiest if you have a solid foundation in 1 and 2, since a lot of it is a natural extension of the basic principles from 1 and 2.
-2 is probably harder than 1 just because it gets into more advanced concepts? improper integration, taylor series, solids of revolution (at least i think this is covered in 2 and not 1). i could see 1 being harder for people if the main sticking point is just the concept of a limit, though.
i suppose that would be 2 > 1 > 3, but i don't think anyone's calculus learning experience is quite like anyone else's.
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