On Trey's Fender Deluxe Reverbs he uses a 2 button switch to change amp channels. It has 3 chords coming out of it. Does one power the switch and the other 2 connect to the amp?
As far as I know that is a relatively simple AB style box. It is mentioned on the rig diagram page on strangedesign.org and you can see a picture of it. I don't think the third cable you speak of is powering the device as I have never heard of a instrument cable powering anything. Think of it this way. One signal in, two signals out as they are switchable (A or B) some of them allow you to use both channels at the same time (A and B). On a standard deluxe reverb however you cant do this without some type of mod as the inputs are out of phase (I think thats what its called). You get either the regular channel or the reverb/vibrato channel. Try any used AB type box, I got one for around $30 and its powered by 9v or power-all device.
Some AB boxes can make unnecessary noise or reduce your volume slightly so if you must have this get a decent one. Analogman sells them, a bit pricey though. I use a very old one made by Nobels and it actually has a boost/level knob for each channel in case you want to get unity gain or a slight boost on one or the other.
If I am mistaken someone else will chime in and get you the correct info.