Multiband compressor for Mac ? (better than C3)

i presume reaper OSX has mac versions of its native plugins with it. You can probably copy the .dll files and get them to work in any DAW

ReaXcomp is actually quite good
 
.dll files are a Windows thing. You want to look for Components if there indeed are native plugins with Reaper in OSX. Just so there won't be a misunderstanding.