This question already has an answer here:
Is there any place where I can download precompiled GTK3 for windows?
The GTK site only provides binaries for the 2.22 version.
GTK+ for Windows in generated by volunteers... So release dates are not synchronized, and depend on the availability of the people working on it. Once they are done, it will be available on the GNOME FTP servers.
And it seems the volunteer behind it is stopping his work. He gives a few hints, like using the openSuSE build service that provides gtk3 components built for Windows (MinGW).
Update: see my more complete answer to that question.
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