A great number of songs would be covered by having one or two independent parts. There is always the need for 3, but then there could be one with 4 or 5 !
Instead of creating a completely separate 3x6 mode, the following suggestions seem to be a lot more easy to implement.
The general use case scenario would be something like this :
You have recorded 3 parts, and played parts 1, 2, 3 and now want to play part 2. As of now, you need to select parts 4, 5, 6, 1, and finally part 2. It would be used to alter the sequence of already recorded parts, not to record new ones.
First possibility (more flexible?):
Allow a direction reversal of the next part selection. A new entry in the playing slideout menu, perhaps “Sel Prev Rec Track” could be added. It should be active only from the slideout menu and not affect the working of the normal Next Part (middle) button in the 6x6 main screen, which will always select the next “sequentially numbered” part.
2nd possibility (slightly less flexible?) :
If you know beforehand the number of parts your song will have, it might be possible to include it in the song settings, something as “Max No of Parts”. So you could save empty songs with any number of parts to use them as templates . This setting would limit the reach of the present “Select Part” button in the 6x6 mode. So from part 3 you would go to part 1 and then part 2. (3 presses instead of 5).