Playing a 3 note chord on GarageBand performs an UNDO function
In iPadOS 13 undo and redo are three finger gestures. Undo is supposed to be a three finger swipe to the left and redo a three finger swipe to the right. The movement to or from a three finger chord certainly may be recognized as a quick swipe. Try disabling gestures in Settings. Because this is in beta the location of settings like this may change between releases. I'd look in Settings->General and disable gestures to start.