Re: C-3 custom
I asked Seymour this question before "why don't you offer pickups in A3 and A4", and he kinda shrugged his shoulders, and suggested that Alnico 2, 5, and Ceramic are the only ones that really matter. After doing my own magnet swapping, it's apparent that he's basically right, although 3 and 4 have some use.
Both tend to sound colder and slightly lower in output. My C-4 ended up being like a C-5 with a more hard, less bouncy feel under the fingers. The good part was that the mids increased and the bass got smaller, so it would be nice in something like a 335 or Explorer. The A3 kinda sounded like a thinner CC, which isn't bad either.
As long as you know how to pull a pickup apart, and put it back together, it's kind of fun to see how the other magnets sound. I still like A5 and A2 the best, tho.