I've just learned that there is a workaround for PowerCore plug-ins. If you have TC PowerCore, go into the Syetem settings/Plug-In Settings and remove the PowerCore plug-ins from the search path.
You still won't be able to use PowerCore plug-ins in GS3. However, if you're using GS3 in ReWire mode and your host uses the PowerCore plug-ins, the system should be happy with this arrangement. Hopefully the next update to GS3 will be able to support PowerCore, but until then this workaround should help you out.