Symbolism, Imagery, Allegory
Triple Threat
We're going to hand the mic over to critic Helen Nebeker, who argues that the three legs of the stool are like the three aspects of the Christian Trinity (God the Father, God the Son, and the Holy Spirit) and that the use of the stool to support the black box represents the manipulation of religion to support collective violence. (Source)
And hey: there's enough corruption of Christian iconography and symbolism (check out our discussion of The Black Box for more) in this story to make that a super-credible reading.
But we also want to point out that the stool could represent other trinities—Norse mythology has the Three Norns (goddesses of fate) and Greek mythology has the Three Fates (who are also, um, goddesses of fate). These ladies determine the fate of both mere mortals and of the world as a whole...kind of how the contents of the black box determined who gets stoned to death. Another reason why the stool might tip its hat at an older tradition? It may serve to underline more generally the ritualistic significance of the lottery as a holdover from generic Ye Olden Days.