I asked a girl I have previously slept with to get drinks and she told me she was already drinking with her best friend (who I've never met). I said ok that's cool and planned on doing something different, but she texted me back immediately inviting me to meet up with them.

When we slept together last she briefly mentioned wanting to do a threesome at some point.

I don't want to go if it's going to be a waste of time.