I guess it comes down to the way I perceive the fundamental natures of the characters. Although I could see Adam participating in a three-way (or more-way), I can't see Aziraphale and Crowley as being up for it. For all that they're immortal and have literally seen everything under the sun, they're each of them also curiously...reserved and private, each in their own ways. It's amazing how their world always, in the end, boils down to their two-being microcosm. And that's one of the reasons I adore them so much :)