Prepare the vegetables: Add the diced cucumbers, onion, and bell peppers in a large mixing bowl. Combine with salt and cover the mixture. Let it sit for at least 6 hours or overnight.
Rinse and drain: Rinse the cucumber mix thoroughly with water and drain again.
Cook the relish: In a large stock pot, combine the vinegar, sugar, and spices. Bring to a boil, then reduce the heat to medium heat and simmer for 5 minutes. Add the cucumber mixture and return to a boil. Reduce the heat back to medium and simmer, uncovered, for 10 minutes.
Can the relish: Ladle the hot relish into hot, sterile jars, leaving ¼ inch of headspace. Wipe the rims clean and seal tightly with lids. Process the jars in a boiling water bath for 10 minutes.
Storage: Sealed jars can be stored in a cool, dark place for up to a year. If the seal is broken, store the relish in the fridge for up to a month.
Enhance Flavor: For best results, let the relish sit for at least a week to allow the flavors to meld and the vinegar to mellow. This also gives the relish time to absorb some of the liquid.