I like my best-friend’s bestfriend

I (20f) have a best friend of five years (20f). A year ago my best-friend met this guy (20m). I too know this guy and have been in the same circle for around three years. My best friend and this guy have gotten closer and closer. They are not romantically involved or interested by any means. For the last couple months i have started to gain feelings for this guy. Mind you my best-friend and this guy are basically besties now. Recently he added me on social media and we have been texting back and forth. This made me more delusional and made my feelings stronger. We flirt over text and online. I told my best-friend about it and she just kind of brushed it off. I dont know what to do. I really like this guy but dont wanna mess things up. Helpp mee