Okay, I looked at the wrong conditional statement and fucked that up!
Here are the conditions for getting the threesome in Episode 8:
you must be on the Sage path
you can't be a CHICK
you can't be on the Maya/Josy path
you can't have gotten the masturbation scene with Jill in Episode 7
If you didn't have sex with Quinn in Episode 6, a lot of the scenes and conversations before the sex scene itself are truncated, but you
can get the threesome if the other conditions are met.
EDIT: Oh, about that sex scene with Quinn in Episode 4, there is no test for that in the entirety of Episode 8, so that is NOT a prerequisite for the threesome. If you got that information from some walkthrough, it is incorrect. However, it is required for the sex scene with Quinn in Episode 6, and will give you a bit less of an experience in the threesome scene if you haven't done that, but won't affect the sex itself, except for one sentence.
Confused yet?