Now I know this probably seems not worth pursuing, what with the end of Sonar/slow heat death of the universe,  but in a reinstall of up to date SPLAT (just before the Bad News dropped) TH3 went AWOL. Annoying as I'd got into using it. Where is it? Any thoughts.Thanks in advance 

When a plugin "disappears" it means one of four things:
1. The DLL is physically gone. Look in \program files\common files\vst3\overloud for TH3-64.vst3 to verify it's there.
2. SONAR is not scanning the DLL. Make sure the above folder is included in the scan settings. 
3. The DLL is being scanned but the scan failed. Open the plugin manager and click on "show excluded" to see if it's been placed on the exclusion list. 
4. Plugin manager has placed it in an unexpected category. This has happened to me before. I found the "missing" plugin by telling SONAR to sort plugins by manufacturer.

Thanks, it's where you said it is but not being spotted. Hmmm

so I redirected the plug in manager to it, it saw it, installed it, I inserted it and Sonar crashed, one of those Sonar then can't reopen without a restart cashes, complete with do I want to report this to Sonar option (not much).  I'm away from the machine for now, though, so will have another go later.  
 
Thanks though Bitflipper