It wouldn't open up the opportunity for three if you are counting the concert yard as one like I think you are, because that concert yard is going nowhere soon. They use it far too often. In all likelihood, once backstage buildings would have to be moved around, it could maybe open up room for one near Despicable me, down that side road. I don't think they have much desire to put an attraction there right now given recent improvements to that area, anyway.
As far as the Soundstage 33 plot is concerned... they could use that now if they wanted. They won't though until 100% out of expansion room because, like the concert yard, they use the building way to much for events.