I am the hinge in a poly MFM Vee. My boys are best friends (and were before I ever met Dude) and we do enjoy the occasional MFM threesome (where I get all of the attention ) even though they are both straight. (We also enjoy a foursome once in a when the opportunity arises...)
We are not poly-fi, however...MrS has never seriously pursued any outside relationships, Dude is seeing Lotus (and she and I are...exploring).
To answer some of your other thoughts:
In terms of "balancing"...MrS, Dude and I live together so we get lots of "all together hanging out time" as well as one-on-one time with each other.
With regards to "going out in public"... around home we are fairly circumspect (like we have a sexy secret...extra fanny-patting and "stolen" kisses); when we are out of town, I love parading around with my boys all out-there and brash (yes, you get all kinds of looks )
We are all fairly independent and are fine with "alone time"...
I have no idea how you could "cultivate" such a situation other than just living your life and being open to the possibilities that arise....you can read my "Journey" blog here if you want to see how it happened for me.
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 female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
TT: poly male, married to Lotus, FB with JaneQ
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe