This makes no sense in terms of watching TV on a computer. You do indeed tune to Channel 3 (or 4, if Channel 3 is a real channel in your area) - but you only do that on a TV, which has two things an iMac G4 lacks: (1) A TV tuner; and (2) a coaxial RF input.
For a computer, all you need is something to get the video output of the cable/satellite box into the computer. Any modern box will have composite, component, and/or HDMI output; and the iMac has USB for input.
But, again building on what Forrest has said, you're going to have a tough time watching TV at an acceptable size or frame rate using USB 1.1, which is all the iMac G4 has.
If you just want to see the TV while you're working on your Mac, it might be serviceable. But if you actually want to turn your iMac into a TV, it won't work well enough.
The only other alternative would be to use Firewire for the video input instead. But - unless you already have a DV camera lying around - any firewire solution is probably going to be relatively expensive.
M