Don't think of these as three "colors" but as three values.....Navy as your dark, pick a red that can be a medium, and the light green can be a light.
Lay the fabric choices out overlapping each other, and squint through your eyes at them to determine how much contrast they have. Or put them in your copier and make a "grayscale" copy, that will show the difference in their value.
Also, just because you will have three color families doesn't mean you have to use only 3 pieces of fabric. For instance, you might use 5 reds, and maybe 3-4 navies, and several greens including very pale fabrics with tiny green print. Lots of different blocks you can make using lots of fabrics.
Jan in VA