So she's basically cucking her own younger self by stealing him from his first love? Bro, I hate to break it to you but your wholesome first love girl is actually a thirsty fox...
Easy detail to miss: this is both characters 3 years younger. In the main timeline, they are 21. This is when they are 18. I didn't expect any time jumps so I personally didn't care to pay much attention to ages. But yeah, this isn't a love triangle, its just their first meeting.