On the topic of Barney, while I'm not a fan and most likely wouldn't buy a DVD set, I agree that the early episodes (and especially the VHS releases that started it all) need to be on DVD. The longtime Barney fans probably deserve the old episodes as much as us longtime Sesame Street fans.
On the topic of Old School Volume 3, somebody uploaded some images of various parts of the packaging and booklet on Muppet Wiki. There's images from the outer and inner back cover. Unfortunately, for the most part we still don't know what bonus clips are included. The inner back cover shows images from segments that we know appear in the episodes (Big Bird and Madeline Kahn, Allistair Cookie introducing Me, Claudias, Harry Belafonte and the Count, Herry and John-John, and Ernie observing the Bert and Ernie statues). The outer back cover does not mention any of the celebrities who appear (in contrast to the first two volumes), but the outside back cover does have an image of Big Bird with R2D2, so I guess one of the bonus clips will feature R2 (and maybe C3PO).
The spine includes Cookie Monster.