I found for Level II that I did not have to refer to my Level I materials. If there was something I didn’t know (presumably from Level I), it would be in the answer to a problem or in the readings, i.e. everything I needed to know was somewhere in the Level II books. Does the same hold true for Level III? Or did you find yourself having to reference Level II consistently, occasionally, never?
Schweser are quite good at covering the background if required. For example, in fixed income pm, they have a few pages refreshing IRP before cracking into the decision of whether to hedge overseas bonds or not.
I didn’t go back to L1 or L2 books. Everything you need to know should be in the L3 materials (of course, they build on previous topics so you need to have some understanding of what happened in the wreckage of L1 and L2). There’s plenty to read without adding L1 and L2 to your study list. Good luck!
I don’t think I referenced my L2 materials at all. With that said, I reference my books for my job all the time, and there are things described more in depth in certain levels’ books.