This is finally working fine. I added beyond compare 3 - and without quotes - and also the full path. Here is an example of one of the lines
text_file_delta compare C:\Program Files (x86)\Beyond Compare 3\BComp.exe
I notice that relative path also works, so maybe it was just having quotes around the path
text_file_delta compare ..\..\..\..\..\Beyond Compare 3\BComp.exe