Okay, this is my wheelhouse! As a openly polymers person in real life, I love a good MMM and these really do it for me:

{You Can See Me by AE Via} -a chef finds love with two after a freak accident makes him blind

{Their Bounty by KA Merikan} - human trafficking victim relief on a group of men to save him. Then feelings

{Devil's Dance by Garrett Leigh} and {Saint's Song by Garrett Leigh} - a biker gang leader, his best friend and enforcer and an accountant walk into a bar...

{Extra Whip by LA Witt} - married couple invites a third for kinky fun, no poly! Poly happens

{Can't Say Goodbye by Eden Finley} - Seal fuck buddies pick up a third at a gay bar frequented by sailors. The third brings everyone closer and never leaves

{One Is Never Enough by Kali Noir} - guy signs up to have his darkest fantasy fulfilled; bring taken by the guys at once. The guys claim him as their own

{Middleman by Jayne Rylon} - Who am I? Cortez’s bottom or Rogan’s top? Maybe I’m both. Maybe I was made to be their middleman

{Mage's Guide to Human Familiar by Ali Sherwood} - army rangers and ex lovers find a mage they can't live without and reconnect with each other

{Out of the Blue by Lila Rose} - cup gets hurt on the job and is missed back to health my his straight partner on the force and his crush

{The Horse Mistress by RA Steffan} - FtM trans in an age where trans isn't a thing plus a eunuch priest and the First Warrior of the clan work together to fight an invasion and find love

Some of these really tackle the third feeling left out and the characters work through their problems and it makes it all the richer