The love triangle between Kelly Ashoona, Emma Nelson, and Manny Santos began in Season 8 of Degrassi: The Next Generation when both Emma and Manny both liked Kelly at the same time. Kelly was in the center of the love triangle. 

Love Triangle History

In With or Without You, Manny and Emma both set their sights on their "single" roommate, Kelly. While in class, Manny and Emma notice Kelly's intelligence, and both consider asking him to be their partner for their self projects. When they ask him to be their project partners, he declines because he tells them he's with Liberty. Manny and Emma are left as partners, even though they're best friends and they already know everything about each other. While working on the project, they both realize that they have feelings for Kelly and decide to go for him. Manny complains that Emma always seems to like the same guys she does, but Emma thinks that they shouldn't do this again. After thinking it over, both girls decide not to go in that direction again.

However, without containing themselves, they rush to see who Kelly has feelings for out of the both of them, and when they open the sock cover handle to their room, they see Kelly in bed with Manny's former roommate Gwyneth. Kelly uncomfortably asks if they didn't see the sign of privacy, being sock on the door. Emma and Manny awkwardly leave and laugh together as they close the door. Later on, after finishing up their project, the girls confront Kelly in the kitchen and try explain everything. He tells them that he is currently dating Gwyneth, but decides to make a rule that he should also not date either of them because they're roommates. Manny and Emma agree to this, and promise each other to stop questioning whom he likes. However, Emma eventually asks if he would date either of them. Kelly reveals that he may be interested in one of them, and then leaves for the library. Emma and Manny look at each other and ask themselves which one, but they then decide that they cannot go that far, and decide to drop it then and there.

In Lost In Love (1), Kelly later reveals to Emma that he likes her, then they go on a date and it is shown that Manny is happy for them.