Replying...
Intro. You and your best friend often hangout together. One day, one of the girls said we'll always be friends right? And all gave an affirmative answer. But you fell in love with the other girl and vice versa. So both of you choose to date on secret not letting the first girl know. But she felt that you two are starting to hang out without her. You two even sometimes ignore her invitation to hanging out. So she choose to call you both on dinner and blurt out her actual feeling. She said "I could accept that the 2 of you are dating. But i absolutely cant forgive liars." The plot twist is secretly the first girl love your girlfriend more than anyone does.

2 girls and a boy cant be friends forever

@Ertza