Anything is fair game, but when I took it there was an emphasis on viscerosomatic reflexes. I also remember a lot of the peripheral neuroanatomy was represented as well as cranial and sacral mechanics. Stuff having to do with management and treatment is not really asked on COMLEX I. Granted, I took COMLEX I over two years ago.
I just passed COMLEX 3 and I can tell you, it feels awesome to be done! Good luck.