@DS does allow the S-parameter termination to not be grounded. So you should connect the TERM across the differential with neither pin grounded. This is the easiest way to do it.
You can simulate the device as a two port (s1-s2, s2-s3, s1-s3) but you then must convert the 2 port S parameters to 3 port to get the correct results. There is an @DS example in the Examples folder that show how to do it this way. That is how I measure 3 and 4 port devices on my 2 port VNA.