Wash cucumbers and peppers. Chop, discarding pepper stems and seeds; seed cucumbers, if desired. Measure 6 cups of cucumber and 3 cups of peppers. Peel and chop onions; measure 3 cups. Combine vegetables in a large bowl. Sprinkle with salt; add cold water to cover. Let stand, covered, at room temperature for 2 hours.
Pour vegetable mixture into colander set in sink. Rinse with fresh water and drain well.
In a 4-quart Dutch oven or kettle combine sugar, vinegar, celery seed, mustard seed, and turmeric. Heat to boiling. Add drained vegetables; return to boiling. Cook over medium-high heat, uncovered, stirring occasionally, about 10 minutes or until most of the excess liquid has evaporated.
Ladle relish into hot, clean half-pint canning jars, leaving 1/2-inch headspace. Wipe the jar rims; adjust lids. Process filled jars in a boiling-water canner for 10 minutes. Remove the jars from the canner; cool jars on racks.