In a large bowl, combine cucumbers, tomatoes, and red onions. In a small bowl, whisk together olive oil, vinegar, sugar (if using), salt, pepper, and oregano. Pour the dressing over the vegetables and toss to coat evenly. Cover and refrigerate for at least 30 minutes to allow the flavors to meld. Before serving, toss the salad again and garnish with fresh parsley or basil if desired.