The generic GLAMOR X.Org graphics driver that does 2D acceleration via OpenGL now has DRI3 support. The GLAMOR library is optionally used by the Intel and Radeon drivers -- or required for the RadeonSI-based hardware. The usage of GLAMOR though is beginning to grow as it becomes faster and better supports various X.Org features.
With this Git commit there is now DRI3 support for GLAMOR, but the DRI3 helpers it provides right now will only work with the Intel DRI driver. The DRI3 support is contingent upon some EGL extensions that are right now only supported by the Intel Mesa driver but will eventually be found with the other Mesa/Gallium3D drivers.