The DRI3 support for Mesa come across as a set of six patches -- four patches make changes to the Intel DRI and DRI common code while the other two patches introduce DRI3 support for GLX, DRI common, and Intel. The sixth patch allows GLX with DRI3 to use the DRI3 Present Extension when available.
With the patches coming from Keith Packard, an Intel employee, only the Intel Mesa driver is hooked up for DRI3 support while namely the Nouveau (NVIDIA) and Radeon graphics drivers don't yet have DRI3 support for their mesa drivers or their X.Org DDX code. DRI3 support for other graphics drivers will come in due time.
The DRI3 Mesa patches for now are floating on the dri-devel mailing list. The Mesa Git code is being branched this week for Mesa 10.0 so the DRI3 support might not land until the following release (either Mesa 10.1 or Mesa 11.0 pending OpenGL 4.0 compliance), which is likely to arrive around February of next year, but that wouldn't be too bad as X.Org Server 1.15 won't ship until the end of the year and this would give more drivers time to implement DRI3 compatibility.