GTK3 support in MATE 1.10 is considered experimental. Since the MATE 1.10 release there has already been a significant effort to improve GTK3 support in the development branch. I imagine GTK3 support will be considered stable in MATE 1.12, for which there is no release date yet.
As for when a GTK3 version of Ubuntu MATE is available, that depends on when MATE 1.12 is released and what we (the Debian maintainers decide) as to when we migrate to the GTK3 builds.