Not sure how that would work, though. In PMD, you and your partner are pretty much glued together almost all the time, except in certain storyline circumstances where they get separated for whatever reason.

And if you just set it where it's only you and your partner, that's not all that hard. Usually that third "NPC" Pokémon you recruited doesn't make the difference. They might help a tiny bit for boss battles, but usually it's you and your partner that do like 90% of the work.