The device you seek is often called a 'TV Modulator' (mention "video/audio in - ch 3/4 output"). They're available in the electronic shops for anywhere between Cdn$8 (surplus stores like Princess Auto) to Cdn$50 (a bit overpriced).
The inputs are usually video (yellow jack) and two audio (white and red jacks), as well as AC power. The output is an F-connector with Channel 3 or 4 (selectable). Some have an S-video input connector, but that's a bit 'smoke and mirrors' since the video has to be immediately converted to composite anyway. Some have another F-connector input for your cable and might have automatic switching (in and out of the circuit).
The harder part is mixing it onto an existing Cable line with the other signals at the same time. The reason it's difficult is that these sorts of modulators don't really have broadcast quality Vestigial Sideband suppression. So the Channel 3 output will probably spill over onto an adjacent channel (I think Channel 4). They'll actually spill over onto several adjacent channels in the worst case.
There are high-end devices that do exactly what you want, but they're in the $100+ range. Also, they are often in the UHF TV band instead of channel 3/4. Google UHF modulators and check the local high-end video stores.
