If you have a three server floating license for ARM development tools, the top of the license file will look something like this:

SERVER this_host  001234567890 8224
SERVER this_host2 000987654321 8224
SERVER this_host3 80abcdef 8224
VENDOR armlmd
USE_SERVER
INCREMENT ...

You cannot "convert" this license into a single server license by commenting out two of the three server lines.  Doing this will break the encryption in the file and result in licensing failure.  The only fields you can modify yourself are the server host names (this_host*) and the port number (default value 8224). Installing your floating license file is covered in more detail in the FAQ: How do I install my floating license? .

If you want to convert your three server license file to a single server file, you will need to rehost your license.  See the FAQ: How do I move my license to another machine?  for more information and a link to the online rehost forms.