I'm afraid that there aren't many songs just with these three notes. to play using just three notes is good for learning improvisation, though. so, to play songs, I higly recommend you to learn at least one whole scale (C-major or G-major are the easiest), then you can play many things. if you're gonna stick with three notes for some time, it's a great opportunity for you to learn how to improvise :] - then, however, I'd recommend you to choose three (or more) notes from a blues scale (e.g. using these A and G and instead of B play C or E1) - then your improvisation, even just on three notes, will sound very well and you'll like it.