Trioshift is another AU created by DustyTheGoat. Instead of characters swapping in pairs, they swap in groups of three.
Toriel takes the Protagonist/Player Role. She falls into the Underground at the beginning of the game.
Sans takes the Empty One Role. He is actually a flower with Sans's face, and the reincarnated of Mettacrit. He also says the world is 'dunk or get dunked'
Undyne is the caretaker of the Ruined Falls, obviously taking the Caretaker Role. She takes Toriel in after finding her weakened by Sans.
Papyrus takes the Recluse Role.
Frisk takes the Judge Role, and you first encounter her outside of the Ruined Falls.
Chara takes the Ambitious Role. She is the 'sister' of Frisk and she strives to be in the Royal Guard. She also often uses knives.
Asriel takes the Idolizer Role, wishing to see his idol, Flowey, in combat. (Kinda weird, as canon Undertale has them as one in the same. Huh.)
Flowey takes the Captain of the Guard Role. He chases Toriel throughout Waterfall.
W. D. Gaster takes the Chaotic Villager Role. They can be found in a secluded area in Waterfall.
W. D. Monster
Monster Kid takes the Forgotten Scientist Role.
Napstablook takes the Royal Scientist Role. He replaces W. D. Monster after he fell into the void. He is also the creator of Temmieton.
Temmie takes the Celebrity Role. She is famous, and has multiple different TV shows. She also is constantly trying to sell Temmie Flakes.
Alphys takes the Monarch Role. She is queen of the Underground, and she has murdered six other children before the events of the game.
Mettacrit takes the Soulless Angel Role. He was good friends with Asgore before they both died.
Asgore takes the Fallen Role. You encounter him at the end of a Genocide Route.