NUnit 3.0 is supported by TeamCity 9.1.x now however you have to install the runner and specify the path to the nunit3.console.exe in the step. My question is where do I copy t
You should have the NUnit console on the each agent where you would like to run NUnit tests.
The best option is:
Add reference to the NuGet package (https://www.nuget.org/packages/NUnit.Runners/).
To restore package you could use "NuGet Installer" build step, see following blog post: https://blog.jetbrains.com/teamcity/2013/08/nuget-package-restore-with-teamcity/
After that you just set path like "packages\NUnit.Console.3.0.0\tools\nunit3-console.exe" from the restored NuGet package.