PostGIS 3.0.0 is planned for release early next week. In the meantime you will find PostGIS 3.0.0rc1 or rc2 available via yum.postgresql.org, apt.postgresql.org, and EDB Windows 64-bit stackbuilder for PostgreSQL 12.
Users on PostgreSQL 64-bit 10 or 11 Windows 64-bit will find installers available on the PostGIS windows build bot - PostGIS windows downloads folders.
I'm in the middle of building other extensions for PostgreSQL 12 for windows 64-bit and will provide binaries for those, so stay tuned.
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