Not really... "Thin air" can count for the required "something" if it's, say, moist enough. 2D displays using walls of mist as the projection surface are relatively commonplace already; 3D could theoretically be implemented as well.
But today's technology can easily produce "freestanding" imagery in empty space by a far simpler means. Just project it directly at the eyes of the audience, by using software that tracks the eyes of each individual spectator and shines the required images directly onto those, customized for each, well, customer.
3D vision is an illusion in any case (even when we're viewing actual physical objects), and the illusion is created by carefully manipulating the imagery falling on our eyes. Doing it eye by eye is merely a minor complication today's computers can easily manage, and probably a lesser complication than those involved in more "conventional" 3D projection techniques.