I've been using Cake Build in a lot of projects lately. Cake is a cross platform build automation system with a C# DSL to do things like compiling code, copy files/folders, running unit tests, compress files and build NuGet packages.
Cake supported running NUnit 2.x, but as NUnit 3 was a complete rewrite with a new console runner and new command line options, it needed to be updated, so I added the new NUnit3 build alias to Cake. It is available in Cake v0.6.1 on. It also supports running NUnit 2 tests, so you should consider upgrading all your scripts to use the new alias.
After I created my Son of Obsidian theme extension for Visual Studio Code, I documented how to create an extension for the Cake team and they managed to add Cake support to Visual Studio Code the same day.
If you aren't already using Cake, give it a try.