George, it took me a while to understand your question but I got it now. The two 2-wire (probably also has ground with it making a total of 3) sheathed electrical cable circuits would be your line power coming in and going out to the garage.
The 3-wire sheathed electrical cable(again probably also has a ground with it making a total of 4) going to the ceiling should have a red/hot conductor, black/hot conductor, a white/neutral/grounded conductor, and a bare or green insulated equipment grounding conductor.
Now look in the junction box for the ceiling light and the junction box for the fan/light combo and tell me what wires you have coming in there and how they are hooked up. Also tell me what wires are hooked to what and to each other at your switch box.
I assume you are trying to put in a stacked lever switch like what is made by Broan and used for bathroom vent/light/heat combos. If I am wrong about the number of wires and the inclusion of grounds then let me know. You also don't say whether this is fairly new sheathed electrical cable, old style sheathed wire, or knob and tube which is why I avoid answering these types of questions and those involving 3-way switches because there are so many variables and ways the wiring can/is run.
Whenever you work on circuits make sure they are deenergized by throwing the breaker and then check it with a tester. If all else fails, get your local electrician to do it.