Just to add to the confusion....

Don't forget that you can use the math->component node to extract just the red, blue or green part of an image.
Set the component to 0=Red, 1=Green, 2=Blue and plug the pic into the Color input.

This also has the advantage that you can combine 3 masks into a single image by giving each maska pure shade of RGB and extract just the part you want.You can even overlap them as long as you make sure that the overlapping bits are correctly coloured (EG where Red overlaps Green, shades of Yellow). This can easily be done automatically in any competent 2D app.
You'll still be able to isolate the individual maps...
Cheers,
Diolma