The "modern" convention for toon shading (or at least what I use) is to convert a Diffuse BSDF (gives gradient shading) to RGB using a Shader to RGB node (only works in eevee), and then converting the gradient shadows into shadow "sections" by using a ColorRamp set to "constant". You can then multiply the resulting "shadow sections" over your base texture(s):

You can change the shadow "influence" by adjusting the mix of the MixRGB node:

This works just the same way if you are mixing multiple textures. Just mix them as you normally would (I just used a Noise Texture as a mask for my example), then multiply the shadows over the final mix:

This might not be the solution you are looking for, but since you have only 3 levels, I would suggest going about it something like this. You will probably have to adjust the greater than/less than values, and possibly the "shade of grey" in the center of the ColorRamp to match your scene lighting.

This is just to get the textures representing shadows - if you want the accompanying darkness levels, just multiply the final result by the ColorRamp values: