New here, but I would venture that, at least in fic, Spock is also Special Enough to get in under Kirk's variable relationship "issues": confirmed heterosexual, confirmed player, confirmed married-to-the-ship, confirmed commitment-phobe.

Overall, my sense of the K/S relationship is that the axes are different, as mentioned above, and who-plays-what-role switches around a little. One is not so much a sound-board or foil or "genius wrangler" for the other so much as both are necessary to one another.

And I would say that the presence of McCoy in TOS is essential, and that the duo is indeed expanded to the trio (though I personally don't see McCoy's role as as easily sexualized as the other two).