My exposure to OT3s started in SG-1 fandom, where there was a long-running Battle Over Jack's Penis (tm Katie_m); OT3 seemed to be one way of solving the problem. Of course, we usually had to destroy the world (apocathreesome!) or strand them offplanet to make it work. Canonical OT3s are much harder to justify, I feel. (Which doesn't mean I won't read them.)

Of course, none of this addressed the sad failure of much of the fandom to include Teal'c in all the sexing, for which there is no excuse, especially since Teal'c canonically got more sex than any of the other characters over most of the show.