Hello and welcome to the Forums!
My husband and my boyfriend were best friends before I got involved with Dude and I really think that that has been a key factor in why our configuration is working for all of us. My boys are both straight, but, while they don't have a sexual relationship with each other, they do love each other and are fine with having sexual encounters with me at the same time (although that is not our usual encounter). I have read a number of women here state that the idea of being with two men who are also with each other is quite a turn-on for them - so you may, in fact, be able to find what you are seeking (not necessarily here but in the world at large)
One thing that you may want to keep in mind as you are looking, however, is that relationships tend to form and grow at different rates. What often seems to happen is that the third person may be interested in both partners but the relationship with one will advance at a different pace than with the other. How would that affect your current relationship with each other and with her? Are you open for each of you dating separately while allowing the other the opportunity to meet and date your dating partners if she felt so inclined? Or is it a "package deal" from the get-go (a harder proposition)?
Just some food for thought.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (22+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe