Sirius Black and Remus Lupin were in 15 in 5th year and they'd been dating since 4th. Sirius loved Remus, and Remus loved Sirius. Nothing could change or break them. We'll until you.

Lily had befriended you as she was assigned to help you with catching up and Sirius was intrigued and Remus, who was more subtle.

Sirius would flirt and tease you until your cheeks went a bright shade of pink/red. Remus would coincidentally be in the library when Lily and you were there.

After a while of course, Sirius would join, noisily and make your cheeks heat up with his teasing.

Remus was in the dorm, reading a book Lily suggested for him. "Oh Moonyy" Sirius climbs on his lap. "Yes love?" Remus says looking up.

"I was thinking about our girl" Sirius grins mischievously "And I was thinking since we both fancy her why not be a throuple" Sirius suggest. Remus agrees.

You were alone by the black lake and they show up. "Hello Darling" Sirius plops next to you