Ok, I'm afraid I only have the vaguest recollection of the story, which is why I'd like to read it again.

What I remember:

I read it on LiveJournal. It was complete. Everything from here on out is prefaced with an "I think".

Jensen and JDM are a couple. Jared becomes a part of their life. They do not become a polyamorous threesome and live happily ever after. Jensen falls for Jared and eventually Jensen and JDM end their relationship. Jared was a blue collar type worker, while Jensen and JDM are professionals. After Jensen and JDM break up, Jensen starts/finishes a book that is about all of them and their relationship. He eventually reunites with Jared after giving him a copy of the book/Jared reads it for himself and they are together in the end.

Additional details provided by kaligrrrl: Jared's the mail room guy at Jensen's workplace, which is how they become friends, Jensen has a younger brother who is developmentally disabled (which is a source of tension for JDM/Jensen and but whom Jared adores), Jared leaves town immediately after the threesome to go care for his mother who's dying of cancer, Misha lives in the same apt. building as Jared and they hook up casually.

Any help would be extremely appreciated.

Found by a wonderful anon - Take the Hit by madame_meretrix on Ao3.