it depends on what the third texture should be. adding a detailmap on one of the tiled textures or on the whole terrain is easy and i think it's described in this thread although it will be hard to filter out this information!

if you want to have a third tiled texture which behaves like the other two tiled textures, a way to control where it should appear is necessary. i think one of the color channels of the "macro map" could be used for that but i would have to check wolfgang engels shader fundamentals again to see how that works.