The NUnit3TestAdapter extension works with the Visual Studio Unit Test window to allow integrated test execution under Visual Studio 2012 and newer. Note you can use the nuget adapter version, which will install into your solution, and makes it easier to run your tests on CI servers. In that case you don't need to install the vsix.
This adapter only works with NUnit 3.0 and higher. For use with earlier versions of NUnit use the NUnitTestAdapter (note - no 3) extension.
Version 3.9 and upwards works for Visual Studio 2012 Update 1 and higher, and Visual Studio 2013, 2015 and 2017. Version 3.13 also works with VS2019
To use Visual Studio 2012 RTM, please use version 3.8 or lower.
NOTE: - EARLY WARNING: Please observe that VSIX test adapters will be deprecated in the upcoming Visual Studio 2019. Please add the nuget adapter package to your solutions instead. See further information here and here