There are no three colors that you can mix every single color out of. That's because of the limitations of pigments, the binding in the pencil, the way the light waves reflect off the pigment. The printing industry often uses only magenta, cyan, yellow, and black ink on a white background because with those 4 pigments, you can mix most other colors closely enough without the expense of buying many different colors of ink. It does not translate as well with colored pencils because of the limitation of layering, human error, and only having certain pigments available in colored pencils.
You will be able to create different values of blue by mixing them with black and white, but you will not be able to mix every shade of blue. There is a big variety of colors and pigments that we call "blue." Different pencils use different pigments. Mixing blue with a little of it's complimentary color will actually desaturate the blue.
I definitely recommend buying the 60 set though. Having more colors to play with is always fun