Start with a 3D shape - either in oblique or in isometric.
Get your 3 tones from using different pressures with your pencil.
Put the darkest (pressing on the hardest) on one side.
Put the lightest (pressing on the lightest) on the top.
Put a middle tone on the front. (press on harder than the top but not as hard as the dark side)
When using coloured pencil, the simple way is to use them in exactly the same way as a drawing pencil.
You can press on hard, medium and lightly with a coloured pencil so you can get dark, medium and light tones in the same way.
Use the same 3 tones - dark, medium and light.
Leave the corner edges white - no colour or tone.