Cut the tomatoes into 1/2-inch dice. Peel the cucumber, then quarter lengthwise. With a spoon, scrape out the seeds. Cut the cucumber strips crosswise into 1/2-inch slices. Peel the red onion and cut into 1/2-inch dice.
Place the vegetables in a serving bowl. Toss to mix. Sprinkle on the sugar, cumin and salt. Stir until all of the vegetables are well seasoned. Serve at room temperature or slightly chilled.