Where do you want your modem physically? If you want the modem in its own room, then an unbalanced 3-way splitter should be fine. Put the upstairs TV and the modem on the 7dB legs and downstairs split on the 3.5dB leg. If it's in the same room as the upstairs HD box, then keep using your existing setup plus a 2-way splitter at the wall in that room.
If you want the cable modem in the same room as the two lower floor devices, then you might want to use a 6dB directional coupler or something and put the lower floor split on the ~1dB leg...then use either a balanced or unbalanced 3-way splitter there.
I have my modem behind two 3.5 dB legs and the downstream power was a little iffy at -10dBmV without an amp/booster. An amp with a passive return put it back to +5dBmV and SNR went from 34 to 36dB. My run however, is quite long...and there's also quite a length of cable inside the house too.