How to say no interested nicely

So my best friend has a visitor from the states staying with him. (Back story.. me n best friend started dating n 3y later he ended things but we stayed best friends. I have a boyfriend 2y n going strong. We won't let anyone call the other best friend but us. And yes I know I'm a ah coz I'm very territorial of my best friend😅).

Now here's the thing. She's staying for a couple of weeks and they're going places n doing stuff together. Cool awesome but his asked if we can all meet up (3of us) as she wants to see me too. Bear in mind she's a acquaintance at best n even if that. I've literally no intention of being a 3rd wheel and really don't care about seeing her. Let alone hearing all the awesome things they've been doing together. Side note: He doesn't like hearing what I've been up to when my bf has visited.

Now how can I say I don't wanna be around ya both coz i don't wanna be a 3rd wheel or have all the fun stuff rubbed in my face🤔plus I don't really care for her😅. I don't wanna sound harsh but if I said what I really felt it would definitely be worse then I said here.