Scooped up from a comment I left on another blogger's post:
Threes are everywhere. Supernaturals may be at in the limelight currently, but I'm of the opinion that triangles are a staple of conflict generation for writers of all types. Examples that spring to mind are Galinda and Elphaba seeking Fieyro's attentions in Wicked, Booth and Hacker at romantic odds for Brennan's feelings on Bones, and, finally, Marigold and Faye reaching the awkward conclusion that they both fancy Angus in the web comic Questionable Content.
Besides the ability to create tension and strife for the characters to deal with, three is an aesthetically pleasing number stemming from the Fibonacci sequence. It is frequently referred to in the visual arts as the Rule of Thirds and tends to be a basis for the creation of all types of art. We see it repeatedly in writing with three acts in a play, three lovers in a triangle, or three heroes in a team.