Ever Oasis producer explains why the game lets you battle as a party of 3

A portion of a Nintendo Life interview with producer Koichi Ishii...

NL: Like Tri Force Heroes, Ever Oasis features parties of three; how did you arrive at that number? How does having three heroes play into the game’s combat and puzzle solving?

KI: In an RPG, at the very least your party is composed of a fighter, a mage, and a cleric. I was very aware of this when making Secret of Mana. Although back then three characters was already really pushing at the limits of what the hardware could handle…

Also, for the system of switching between characters, there is a big difference between having three characters e.g. (C) →A←→B←→C←(A), and having four e.g. (D)→A←→B←→C←→D←(A) in terms of how much confusion it causes and how long it takes. I wanted players to be able to change characters intuitively and without any stress, so I decided this triangle system would be best.