We tried a few steps to get the Upload Trimble add-in into SS3. We installed the TrimbleLinkEngineInstall.exe located at C:\BentleyDownloads\ir081109493en\Trimble Link Engine Software\Install then restarted the program and it still isn't listed in the Application Add-ins.
Then we tried to force the addin into the program by adding the Upload.3.0.DLL into the registry key at HKEY_CURRENT_USER\Software\Bentley\InRoadsGroup\Applications\Version 8.11.9\inroadssuiteat.dll\AddInApps but this still didn't add in the Upload Trimble tool.
We will continue testing in SS3 but this could be a defect that needs to be logged.