It's commonly called an RF switchbox, but you don't really need a vintage one today. You can get an 75ohm to RCA adapter plug at Radio Shack and plug that directly into your tv's cable input or the cable in on a VCR. And then you tune either one to channel 3.
Obviously, you don't want to unplug the cable tv all the time, so add a 75 ohm switcher, which Radio Shack should still carry and you should be all set. :)