If it really is 3 hours they could give all the PS3 and 4 exclusives their just desserts. I mean I'm sure Vita has some unannounced stuff too.
PS3="We're still supporting it 10 year life cycle etc. etc." then show GT6 (gameplay and numbers of cars and such), Puppeteer (gameplay), Beyond (gameplay), LoU (just mention so people don't forget with E3)
Vita="Most powerful handheld ever and perfect companion for PS4 etc. etc." then show Killzone and Tearaway gameplay then trailers for a couple new games
PS4="We have built the most powerful next gen console which will create new experiences for gamers through social media, cutting edge hardware, and the rich first party titles that you can only get on Playstation" mention no DRM and no always online (really gotta make that clear if they are the good guys) then get to a live demonstation of sharing and streaming on a game already shown. Show KZ MP, Infamous gameplay, Driveclub social race and how that works, and a Knack level. Show Destiny gameplay. Finally get to the new games. New Naughty Dog IP, new SSM IP, and at least one other game shown if only a teaser from another first party. Of course not exactly in this order as they should mix up the new with the already announced but at the end those should all be checked. At the end show the console case itself because people will be glued until they do that because for some reason we love to see boxes, but they will still pay attention and take away everything you have before that. It's a great way to play the tease and hold it until the last minute before maybe mentioning a few of the parts (how many ports, faster USB, how large, etc.)
So I guess PS3 should come first and spend 45 minutes on that, then 30 minutes on Vita, and finally 1 hour and 45 minutes on PS4.