A primer to "Triad Verse", a triad-normalized universe than can be applied to any fandom. A good place to start if you're new to Triad Verse.
What is the triad verse?
The triad verse is an alternate universe (AU) where humankind has developed to normalize triads (relationships consisting of three individuals), rather than couples. Couples can and do form, but they are considered "incomplete" relationships. In most versions of triad verse, couples are unable to marry, but triads are allowed to marry. Bisexuality is also normalized, while monosexual people are less common and more marginalized.
The following is a list of some of the headcanons I and others have come up with regarding this verse. Feel free to use as many or as few of these headcanons when writing your fic. If you'd like us to find your fic, tag it "triad verse" here on tumblr and "Triad Verse" on AO3.
Though fics containing triad verse have been published perviously (Until, by gala_apples; possibly others that I'm not aware of), the first post I made can be found here.
Single sex triads: Ptera's thoughts | Penthea's thoughts | Pilgrimkitty's thoughts
MMF vs MFF triads: Ptera 1 | Penthea | Ptera 2 | Pilgrimkitty
Family structure: Ptera 1 | Panthea 1 | Ptera 2 | Penthea 2 | daddy-klaine
Historical: magdalyna on nobility | rainwasheseverythingaway on religion | More Western Religion
Engineering/City Structure: Ptera | Magdylena
TV shows meta | Movies meta
Fandom specific meta
Fairy Tales: Penthea's Beauty and the Beast meta | Ptera's Snow White | Rain's Disney meta | Ptera's tag
Teen Wolf: Ptera | Pilgrimkitty
Vampire Diaries: Pilgrimkitty
Harry Potter: Pilgrimkitty
Glee: Penthea's Finn/Puck/Kurt preview
Teen wolf: Ptera's series | Ptera's Scallison snippet | Ptera's Sheriff/Melissa/Peter snippet
Avengers: daddy-klaine's snippet