When water is boiling, add kale, kohlrabi, and turnip leaves, and cook until tender, but not mushy, about 2 minutes. Pull out the hot, dripping leaves with tongs, and put directly into a blender or food processor. (Don’t drain the pot; you’ll use that same boiling water to cook the pasta.) Add some of the olive oil. Blend into a fine, thick green purée. Taste, and adjust seasoning with salt and pepper, add more oil if needed, and then blend again.