If your MainStage audio isn't the right pitch, there are a couple things to check:
- Select MainStage 3 > Preferences > General > Tuning- check that this is set to 440Hz
- Select MainStage 3 > Preferences > Audio > Sample Rate- check that this is 44.1 or 48. If you are using an audio interface, you'll want to make sure this matches your audio interface- 44.1 is the default for most of them.