There is a VST3 readme included in most of the installers.
And if a plugin still has the old VST3 included, the VST3 option is unchecked in the installer (with another hint to read the VST3 readme).
Already save to use with VST3 are the following plugins:
Hive 1.2 (coming soon)
Effects (the most current release versions):
Twangström (the public beta)