When you do the MSB/LSB/PC (program change) - send them on the MIDI channel that matches the Part you want to change.

Also, make sure you're not in single-channel or hybrid mode or the synth won't be able to tell which Part to swap out (since they're all "merged" on the same MIDI channel in either of those modes).

When you're using MSB 63 - this will, by definition, take the 1st Part of that Performance and place it in the Part you designate by the MIDI channel used when you send the MSB/LSB/PC messages. So you can go to any Performance (Presets, User, or Library) and look at what Part 1 is. That will be what gets swapped into the Part of your choice when doing the MSB=63 single-Part Performance feature.

Say you really want Part 3 of some Performance to get "swapped in". There's not an MSB/LSB/PC method to do that. What you'll have to do is take Part 3 of that donor Performance and save it into a new Performance (user or first user and promote to a library) where what was once Part 3 is now in Part 1's slot in the newly created Performance. Then you can use MSB/LSB/PC of the NEW Performance in order to get what once was Part 3 into the current Performance's Part of your choice (again, by using MIDI channel to designate).