Not an Airtronics guru but I did stay in a Holiday Inn last night
If I'm not mistaken, the MX-3S is the synsthesized FM version.
Now they had the MX-3FHSS 2.4Ghz radio and now they have a newer version of it with the FG designation. As far as I can tell, the only difference between them is that the new version has a higher power signal output.