Reference for GIMP 3.0 API
The 3.0 API is at the last stage of development: release candidates. It is considered nearly stable though there may still be last minute changes before GIMP 3.0.0 release.
Since bindings are generated from introspecting the C library, the above documentation may be enough for all bindings (Script-Fu though is a special case!) and you should be able to extrapolate the equivalent functions according to your target language syntax. Nevertheless, some projects exist for generating language-specific documentation from the same source.
For instance, Python-specific docs are made available here:
Note: if you know of more binding documentation of our libraries, hosted by third-party projects, please report them