DRI3 was conceived back in Germany last September by Keith Packard and Eric Anholt at XDC2012. DRI3 still revolves around using POSIX file descriptors for passing kernel objects between the X Server and the application. As part of the proposed DRI3 is also the swap extension for properling swapping the screen contents in a synchronized manner.
DRI3 was previously talked about back in DRI3 Was Just Proposed On The Fly From XDC2012, Planning Out The DRI3 Implementation Details, Keith Packard Outlines "DRI-Next" Plans, and Passing File Descriptors Over The X Protocol Socket. That's plenty of reading for those not up to speed on the "DRI3" plans. Keith calls this work "Even Better Direct Rendering."
While there isn't any new code to share today, Keith Packard has recapped his latest DRI3 plans on his blog for those interested in this next major DRI update. It's worth reading and there's also his LCA2013 video (MP4) about the subject for those interested.