I have the RB3 for Wii bundle, and I have Xbox guitars, microphone, and drums. If I purchase a Wii RB3 MIDI adapter, can I use my Xbox instruments on my Wii?
No, the MIDI adapter allows instruments that output MIDI signals to be used as game controllers. None of the current game controllers output MIDI, the purpose of the MIDI adapter is to allow you to use real electronic instruments like a standard MIDI keyboard (as in piano keyboard), or a standard electronic drumset as controllers in RB. The RB3 Squier "Pro Guitar" (which is actually a real electric guitar) also requires the MIDI adapter to use. (The RB3 Mustang Pro Guitar does not require a MIDI adapter, but still wouldn't work on a Wii.)
If it's any consolation though, the microphone should work, since it should just be a standard USB mic, which works on any system.