A month or even a few weeks can be a long time for something like sex/dating/seeing someone. Everything can be different within even 10 days. Or less. Would be interested just in that, what it's like between you two in 3 weeks. I wouldn't worry about it at all, but if it was me, I would consider going alone and hanging with the girl and her friend. it's not that seeing someone else is some cardinal sin, but to intentionally flaunt it...is a little much. At best, you will have to address that issue. Happened to me before, and her seeing me with someone else, was a little mini drama I didn't need.
but like me, it never hurts to know a few women whom you are good with to roll with you on things like that, just in general. but in this case, it's egregious.
applicable only if sleeping together/enhanced intimacy beyond like a quick kiss