^^^
I think that that will be the answer:
From here:
http://avid.force.com/pkb/articles/en_US/compatibility/en343311Pro Tools 12 and above require plug-ins to be AAX format and 64-bit.
T-RackS 3 plug-ins are VST 32-bit, VST 64-bit and RTAS.
The good news is that you can install T-RackS 5 Custom Shop, together with the Authorization Manager and Custom Shop programs (both of which are 32-bit, so don't go to Catalina). Run Custom Shop, login and, in the T-RackS section, select [Gear] and "Restore my gear". You should then be able to use TR5 editions of your TR3 plug-ins fully; you can also demo and try the others.
T-RackS 5 plug-ins are: Audio Units, VST 2, VST 3, AAX.