Thanks...but why is there 3?
depends on the stand / mount.
5/8-27 is the standard size for most mic bars & shock mounts
3/8-16 is what a lot of photography mounts use (hence the adapter issue)
1/4-20 is also used with photography mounts (the air cushioned light stands, like i use, have a 5/8 stud with 1/4-20 thread on top)
so i have a 1/4-20 > 3/8-16 adapter screwed on to my stand, and then a 3/8-16 > 5/8-27 screwed onto that (or into the mic clip) for the main mount onto my stand. also, my rowi clamp uses a 1/4-20 thread for the swival mount part of it.
my vert bar has 3/8-16 bolts through it that i screw on 3/8-16 > 5/8-27 adapters in order to use my at mic mounts, or just use the ones that came with my superlux shocks (another reason i really liked these) and not have to ever thread anything in and out of the plastic threads, since the adapter stays in.