I don't often rec stories, but I am now.
It's a threesome: Snape, Harry with Draco.
It is done with a light, gentle hand,. It is filled with misunderstandings and lots of laughter yet underlying is the rather serious problem of how a ménage à trois work.
zephyr_macabee found the perfect lines from the story to sum it up:
"Because-I'm-I'm a total hypocrite," [Harry] finished lamely.
"So what? Snape's a voyeuristic pervert and I'm a manipulative brat. We all have our little flaws," Draco told him.
Go read "And Malfoy Makes Three" by
the_con_cept http://asylums.insanejournal.com/snarry_games/183070.html P.S. Set a fair amount of time aside for this one: it's over 40K words long!