AmiReiUsa

AmiReiUsa is the poly ship between Usagi Tsukino, Ami Mizuno and Rei Hino from the Sailor Moon fandom.

Manga
Ami tells Usagi that on her bus ride home, she gets to see a very pretty girl, so Usagi joins her. When Usagi sees the girl, Rei, she is also taken aback at how pretty she is. After Sailor Mars is awakened, Usagi, Ami and Rei regularly hang out together. Later they attend a fancy party together in hopes of finding the Silver Crystal.

90's Anime
Jadeite challenges the three Sailor Senshi so that he can kill them for good. The three arrive at the airport he indicated. Jadeite sends the airplanes after them, but by working together, they are able to stop him. Sailor Mercury uses Shabon Spray to fog his vision, Sailor Mars uses Akuyo Taisan on Jadeite to make the planes chase after him, and Sailor Moon finishes the job with Moon Tiara Action.

Sometime later, during the summer, Ami, Usagi and Rei take a vacation in order to train. They hike up a mountain, and then it starts to rain, but they are guided to a house by a girl, which happens to be the pension Rei booked. While they're there, they go to play on the beach and have fun. After resolving an incident involving the master and daughter of where they are staying, the three girls play with the daughter on the beach.

One day, Nephrite sends out his strongest Youma to attack the Sailor Senshi. They are twin Youma bound by friendship, and they attack the Senshi with the exact same techniques as them. Sailor Mars, Moon and Mercury all get hurt, and the Youma then argue over who gets to deal the finishing blow. The Senshi take this opportunity to attack them. Mercury uses Shabon Spray to coat Sailor Moon's Moon Tiara Action, which is then combined with Mars's Fire Soul. The super-charged attack kills both Youma.

Fanon
The ship sailed due to Ami, Usagi and Rei being the first three Senshi to join together. They had 15 episodes together with just the three of them before Makoto was introduced. As a result, they had a lot of screen time that made room for the ship to develop easily. Most notably, episodes 20 and 21 helped fuel it. Episode 20 because it was refreshing to see the three of them have fun on a vacation and episode 21 because it was the power of their friendship and combo attack that destroyed the Youma.

The three of them make up a pleasing colour scheme. Sailor Mercury is all blue, Sailor Mars is mostly red, and Sailor Moon's outfit consists of both blue and red. Also, the three make the primary colours. Sailor Moon's hair is yellow, Mercury's outfit and hair is blue, and Mars's outfit is red.

Their attacks and styles go together too, making an efficient team. Mars attacks offensively and doesn't hesitate to attack, while Mercury is the brain of the group and uses defensive measures. Sailor Moon is in between, sometimes being reluctant to fight an enemy and often running away, but always deals the final blow. Their three elements of fire, water/ice and light could potentially go together.

All of these things are why people like the ship. Despite this, it is very uncommon in the fandom, as with Sailor Moon poly ships in general. The ships that only involve two out of the three are much more popular.

Fandom
FAN FICTION

Trivia

 * All of their Sailor names start with M (Moon, Mercury, Mars).
 * All three have a different blood type. Usagi has type O, Ami has type A, and Rei has type AB.

Variations

 * AmiRei refers to the ship just between Ami and Rei
 * AmiUsa refers to the ship just between Ami and Usagi
 * Inner Senshi refers to the ship between the three, Makoto Kino, and Minako Aino.
 * ReiUsa refers to the ship just between Rei and Usagi
 * Sailor Senshi refers to the ship between the three, Makoto Kino, Minako Aino, Chibiusa Tsukino, Haruka Tenou, Michiru Kaiou, Setsuna Meiou, and Hotaru Tomoe