I had to look up triad, cause when I spotted that word I got a bit 'huh?' - got it at first confused with triumvirate, of all things...
Turns out the most common meaning for Triad is Chinese mafia, then something to do with music, then a definition 'a group of three people or things closely related'.
I guess what you had in mind, would be some more slightly obscure definitions like 'a way how to fit three people in a bed' or urban dictionary's definition number 4 of equal relationship of three people.
For that, you could maybe just do a fanfiction search of threesome - on Dokuga, it yields just 5 results and most of them have a very decent number of reads, which usually guarantees a good story.
I'd recommend the Twelfth Concubine by Aubrey Simone, incomplete, unfortunately.
http://www.dokuga.com/fanfiction/story/5178/1
Also, all of those threesomes are MA, so all yims look the other way!