If your signal level is too low now, you might take a chance in lowering it by putting a spitter on, which could cause you modem to not work. I'd say try it, and see what happens. Most modems will tell you what their power levels are by browsing to 192.168.100.1 in your browser, but some modems won't.... Anything above 55db transmit power and below 10db receive could cause you some problems.
If you are considering plugging more than one tv into that outlet, I'd say get a two way splitter, and have one side go to the modem, and then plus the remaining connector into another two way so your modem is getting it's signal on a two way and not a three way splitter... sounds kinda silly but it can help.
mrpeabody