Multiple notes triggered by MIDI - help!

I'm having a problem where when I press one key on my MIDI keyboard, three MIDI notes are triggered and recorded in GarageBand. The notes recorded are the noted played, plus the same note one and two octaves below. In other words, pressing middle C triggers both middle C and the two Cs below it in GB simultaneously.

This doesn't appear to be due to a setting on my ancient Yamaha PSR-300 keyboard.

This started after installing Rhythm Jam Pack 3, discs 1 and 2.

I really have no idea what would trigger this issue or what setting would resolve it.

This is time-sensitive - doing a soundtrack for a friend's short film and it's due within the week. Thanks!