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:
Synths:
Diva 1.4.3
Hive 1.2 (coming soon)
Repro
Zebra 2.8
Effects (the most current release versions):
Satin
Presswerk
Colour Copy
Twangström (the public beta)