The best replacement for mbox 3 is mbox 2.

The best alternatives to mbox 3 are rated as following:
★★★★★  mbox 2
★★★★★  mbox 3 pro
★★★  digi 003
★★★  mbox 3 mini
★★★  duet 2
★★  focusrite scarlett
★★  apogee duet 2
★★  duet
★★  apogee duet
★★  fast track pro
★★  komplete audio 6
★★  profire 610
★★  rme babyface
★  babyface
★  003
★  fast track ultra
★  002
★  digi 002
  mbox 2 pro
  mbox mini
  apogee

Your alternative for mbox 3 is not listed? Suggest!
Speaking about mbox 3, one probably shouldn't forget about pro tools 8 le.

Other search terms related to mbox 3 and their ratings are:
★★★★★ pro tools 8 le
You have in mind a companion for mbox 3 and it is not yet here? Propose!



Google Trends

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mbox 3
mbox 2
mbox 3 pro