EreJeArmin

EreJeArmin is the poly ship between Eren Jaeger, Jean Kirstein and Armin Arlert from the Attack on Titan fandom.

Canon
Eren, Jean and Armin knew each other as trainees from the 104th Training. Eren and Armin are childhood friends, who meet Jean upon enlisting as training cadets. The night after their graduation, and before the trainees had decided on their chosen section of the military, (Military Police, Garrison, or Survey Corps) Eren and Jean start fighting because Eren finds Jean's decision to join the Military Police as selfish and cowardly. Since Armin was sitting with Eren, as Eren starts giving a moving speech to the 104th, he hears it all. When Eren mentions his dream is to see the outside world, Armin blushes and is seen shocked at the revelation. This is because it was Eren and Armin's shared childhood dream to see the outside world.

After joining the Survey Corps with Eren, Jean, Armin and the rest of the 104th's top 10 (excluding Annie Leonhart),  the three of them participate in the Survey Corps' 57th expedition, venturing beyond Wall Rose. Not long into the expedition, Armin's squad encounters the Female Titan, which is seemingly highly intelligent and has considerable knowledge about the Military's formation. The Female Titan kills Armin's entire squad, but spares Armin's life after carefully removing his hood and observing his face. As the Female Titan moves on, Jean, alongside Reiner, meets up with Armin and the three pursue the Female Titan. Armin suggests the Female Titan, like Eren, is a human-controlled Titan and is likely hunting for Eren. Which means that as long as she doesn't see them, they'll be fine. Jean is amazed at this clever observation, so he tells Armin that he's always thought he was a capable guy. However, he also mentions that he was initially put off by Armin always clinging to Eren.

When Reiner and Bertolt take Eren away and the Survey Corps go to take him back, Armin is horrified by seeing that Eren could easily be eaten. In the struggle, Eren and Mikasa are knocked from their horse, and Jean is incapacitated as he and the others rush to assist them. With nowhere to run, Armin drags Jean towards a small cluster of trees and prepares to face an advancing Titan with only one sword. He looks to Eren's direction and the two exchange a prolonged teary-eyed stare. As Armin and Jean are preparing for the end, Eren's "Coordinate" power suddenly activates and he immediately turns back towards Armin's direction to see if he is safe. Armin is saved and carries a wounded Jean to safety.

Back at Wall Rose, Eren asks how many people died because he got kidnapped again, and Jean, being an honest person, gives him all the information about the people who died, and the ones who were injured. Eren starts to feel even more guilty, and Armin tries to comfort him by looking at the bright side - there were no losses on their way back. He deduces that the titans didn’t pay attention to them because of Eren. Eren must have controlled the titans, which means he saved them. This makes Jean look at Eren in a different light now that he knows that Eren saved them, and he tells him that he's not sure if he's worth the price, but says that the legacy of the soldiers who were killed depends on Eren now. That's when Eren thanks Jean because he says that thanks to his words he doesn't think he has to hesitate anymore, and that he now knows what to do.