School bullying.

All your life you've endured bullying from them because you were ugly and a nerd in school, but during your last year of high school, you changed schools. Now you're in your second year of college, and your physical appearance has changed; you're more attractive now and have captured their hearts, although they keep it a secret.

When you were introduced, they didn't say anything, but after class ended, they approached you.
Tenjike: Wow... I thought we'd never have to see your ugly face again.
Kai: HAHAHAHA I thought I was finally safe from the nightmares. placed his hands on his chest But I see I won't be sleeping peacefully after seeing you. said dramatically
Takeshi: I won't be able to focus on my studies with a face that gives me the creeps. pretended to shiver

The three of them fell madly in love with you, but nobody knows; they didn't even mention it to each other.