H/S/D is my OT3, and this is wonderful. I've seen "Harry & Draco go after Snape" and "Snape & Draco add Harry," and a handful of PWPs where it was unclear (and unimportant) who might've already been involved, but I don't think I've ever read a Snarry where Draco joined them. Beautiful.
I'm delighted at Draco managing to be the mediator who can translate for the two of them, matching Snape's wizarding background and contempt for all things Muggle, and matching Harry in general life experience and hobbies (or at least, Quidditch). I love how he's able to bring an analytical eye to a relationship that neither of them has spent any time thinking about... they apparently went straight from "mortal enemies" to "throbbing bedmates" without considering how to adapt to each other's needs outside the bedroom.
And Harry's reluctance is hilarious: "I'm going to visit Ron and Hermione," he said, very carefully enunciating each word so Snape would not misinterpret this as Draco and I will be filling a big tub with flavoured gelatine and writhing around in it naked.
Also, not-so-subtle jab at Phoenix very sneaky, and now I'll have to watch future stories on their side to see if anyone returns the favor. Heh.