One thing I noticed during installation was that the default folder chosen by the Breverb installer was "C:\Program Files\Cakewalk\Vstplugins\TH2 Producer" and the one chosen by the TH3 installer was "C:\Program Files\Cakewalk\Vstplugins\BREVERB SONAR!
Fortunately I spotted this and pointed them to the correct folders. Having said that there was no TH3 folder there but in C:\ProgramData\Overloud\TH3 so I created one in C:\Program Files\Cakewalk\Vstplugins and after installation I see that the TH3-64.dll file has been copied there.
I am now testing old Sonar projects to make sure that they still load OK.
Edit: All seems fine both in Sonar, S1Pro and also tried Cantabile :)