Specific Dean/Sam/OMC fic
I'm sure this was posted on AO3 before but now I can't seem to be able to find it. Dean has been struggling with an attraction to his brother so one night he goes to a bar and picks up a guy thinking he can get it out of his system. But the guy (I think his name was Matt) is kind of perceptive and gets the feeling that Dean is harboring some strong feelings for someone else. Anyway they're at the hotel room and Sam walks in on them. The OMC realizes Sam is also having feelings towards Dean and talks the two of them into a threesome (he doesn't realize they are brothers, he thinks they work together). I know there are some angsty feelings between Sam and Dean thinking the other might not really want the other. Then there was either a sequel or another chapter where it turns out the OMC had made a demon deal so Sam and Dean save him. They all have sex again and Sam and Dean decide to take the hunting with them. Dean and Sam end up in a relationship with the OMC as a semi-regular 3rd.