Summary:

Clint, Bucky and Natasha hooked up on Friday the 13th and later entered a steady polyamorous relationship.
This is not the story of how that developed. This is a gratuitous night of them happily expressing their affection towards each other, at a later date when that relationship is established.

Porn, guys, I am talking about porn.

This is porn. All of it. Enjoy.

Here, on AO3.
 

(Edit: Terribly sorry about the weird formatting on this entry earlier. Hopefully fixed now.)