Prepare the Vegetables
Wash the cucumbers, tomatoes, and onion.
Slice the cucumbers thinly, cut the tomatoes into wedges or slices, and thinly slice the onion. Place them in a large mixing bowl.
Make the Marinade
In a small bowl, whisk together the vinegar, olive oil, sugar (if using), salt, pepper, and oregano or dill until well combined.
Combine and Marinate
Pour the marinade over the vegetables in the mixing bowl. Toss gently to coat all the vegetables evenly.
Cover and refrigerate for at least 1 hour to let the flavors meld. For the best flavor, marinate for 2-3 hours or overnight.
Serve
Stir the vegetables before serving to redistribute the marinade. Serve chilled as a side dish or salad.