Thanks kasky1.
I have an alternate for the amp XTC (the problematic vst3). I have the Native Instruments Guitar Rig 7, thus I am not really depended on the Rosetta. I have also taken up with the developer of amp XTC for a solution sans Rosetta.
Shreddage amp XTC, developers got back to me. Apparently, while installing their plugin, I inadvertently selected the Mac (intel) version instead of the Mac version, and that resulted in all the confusion about the missing vst3 in Dorico 6. I reinstalled the plugin and now everything is working perfectly fine. Thanks once again to Ulf and the other forum members who offered suggestions to help resolve the problem.