Hi,
I faced the similar issue.
The solution to the issue is to pass argument 'format=nunit2' while executing the tests.
So, the command will look like this:
nunit3-console.exe --labels=All --out=TestResult.txt "-- result=TestResult.xml;format=nunit2" bin\Debug\SpecFlowDemoNUnit.dll
And then the reports generated can be converted using:
specflow.exe nunitexecutionreport SpecFlowDemoNUnit.csproj /out:MyResult.html
I hope this helps.