There isn't anything official. Tonis maintains an "Awesome C3" repo on GitHub: https://github.com/tonis2/Awesome.c3 (curation is all done by Tonis, I don't have any say what goes into it).

Share it on the C3 discourse: https://c3lang.discourse.group or the C3 discord (but I think you already did that?)

Other than that... r/Programming, r/C3Lang and r/ProgrammingLanguages might be considered.