Does your cable modem list the downstream and upstream frequencies? Most commonly, downstream will be very high, like in the 600-700's. That is well into the UHF channels (or cable channels 85+). Channels 2-6 are roughly 50-88MHz. Upstream usually uses frequences below 50.
+3db is good. The recommendation is +15 to -15, although some recommend trying to keep it +8 to -8. You also need to check SNR (Signal to Noise Ratio). It should be 30+. Upstream should be below 55db.