If CBS isn't on the HDTV channel menu, a customer service representative, if you get one that knows what you're talking about, should be able to read off the H/DTV channels they offer. A slight chance they just need to activate that channel on your 3100HD.
Could be there's no CBS H/DTV station available they can pick up and relay via cable, or that they haven't reached an agreement with the station, or other reasons. If there is a H/DTV station you could receive with a suitable antenna, and your cable company absolutely confirms they won't be carrying it, you could buy/use an OTA set-top box to pick it up. Vintage RCA DTC-100s are discounted now. Numerous threads here provide web sites listing FCC-approved H/DTV stations, or pinpoint all local H/DTV stations, with antenna suggestions, with a zipcode entry. Finally, CBS is unique for HDTV because you can also receive it via satellite if it's not available locally OTA. Threads here also detail procedures for getting CBS via DBS. -- John