I had this problem too but the cause was different. I'm using VS2017 with F# 4.0.
Firstly, the console in Visual Studio does not give you enough details why the tests could not be found; it will just fail to the load the DLL with the tests. So use NUnit3console.exe on the command line as this gives you more details.
In my case, it was because the test adapter was looking for a newer version of the F# Core DLL (4.4.1.0) (F# 4.1) whereas I'm still using 4.4.0.0 (F# 4.0). So I just added this to the app.config of the test project:-
<dependentAssembly>
<assemblyIdentity name="FSharp.Core" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535" newVersion="4.4.0.0" />
</dependentAssembly>
i.e. redirect to the earlier F# core.