Sorry, can't really help there, I stopped using SONAR back in 2015 and have never bothered with Cakewalk by BandLab, plus I had the full versions of TH2/TH3/TH-U (although as mentioned, sold now). It shouldn't be to difficult to get it back, although I much doubt copying over from another PC will work in this case, in the case of plugins that only rely on a .dll file it would work, but likely not with plugs that require more than that.
Is there an option to include TH3 in the CbB install? is there an option in Bandlab Assistant to install extras or anything?
You could do a clean uninstall/re install, making sure to note any installation options during the procedure. There are instructions for a clean install post a number of times in the forums, try searching for that. It shouldn't be to hard to sort out, but not personally being a user I can't really help much, I'm sure someone will answer one of your posts with the solution.
Best of luck