Sam and Dean when they first met Adam and something just...clicked.
They felt the same thing they felt for each other for Adam; he was family.
They start ever so slightly and slowly trying to bring and ease him into the same dynamic and relationship they have for each other; faint touches on his thigh while they're teaching him something, hand grazes that might be just a bit too long to be normal when passing cups of coffee, and even offering their bodies on cold nights so they can all stay warm.
Soon enough, it works and Adam is reciprocating those same touches and glances. Glances that are pure hunger and lust.
One cold night, they're all in the same bed doing random stuff; just being close. Sam and Dean both lean in to give Adam a kiss on the cheek but Adam turns his head towards Sam accidentally and they kiss on the lips. That kiss soon turns into a make out session which then turns into their clothes flying across the room. No one knows what happened in the middle, too caught up in each other to.
They're frantically touching each others bodies, panting. Nothing truly happens, just hand and blowjobs and yet they all had the best orgasms that night.