Really? Just three?
Right. White, Middle grey, and black.
Most scenes have many more than three values, of course, so this over-simplification will involve some serious rounding up and down. The image below, for example, will need to be adjusted considerably. The sky is darker than the sunlit yellow building, so it must be middle value, right? But it's lighter than the blue building on the right, which is lighter than the shadow on the street. If yellow building = white, sky = middle and blue building = black, what about the shadow on the street, and the windows and doors on the blue building? Something must be changed.
You could make the sky white, like the yellow building. They would be separated by that thin dark line along the top of the building. Then you could make the blue building mid-value, and the cast shadow black. Problem solved?
I know what you're thinking, "what about the mountain?" It is darker than the sky, but lighter than the blue building. If you make it white, it will merge with the sky, and be gone. If you make it mid-value, is there anything that will keep it separate from the shady side of the street? Remember, you are in charge, not the photo. See that tree on the right that sticks up in front of the mountain? You could round that up to black or down to white and it would contrast with the mountain and the shaded buildings, separating them.
There are other schemes that would probably work just as well. What if you left the sky mid-value and made the blue building black? Or you might make them both middle, and exaggerate the dark lines and windows on the building.
Is anyone still reading this? I enjoy this kind of puzzle, but I'm a Virgo (recovering).
The idea is to find out where more subtlety or specificity is needed. If you can keep the feeling of space with only 3 values, great. If not, then the study has given you the information you need.
Some things are obvious without even making a study. Just thinking about rounding values up or down got me to notice that I like the mountain being just a little darker than the sky, so I already know I'll need more subtlety there. But what about that yellow and red building, on the right? is it middle or light? If you got it just right, would it look like it's in a shadow? I'd have to see it in monochrome to know for sure.
Once you've made any necessary adjustments, start by identifying the shapes that are closer to white than to middle value. Outline them roughly so you'll remember where they are.
Mix up a big puddle of a middle value paint. Use a color (just one, not a mixture) that will get dark enough to represent the darkest darks in your image. Make your supply generous, so you definitely won't run out. Now paint the whole page middle value, except for the whites. While it's drying, assess the effectiveness of the illusion of space and light. How much of the story is told without the darks?
Finally, put on those darkest shapes. How's the feeling of space now? And light? Is anything too light or too dark? Where do you need more subtlety? More specificity? How might the composition be improved (take out that gum drop tree, make the lamppost on the left a little taller?)