In a small bowl, whisk together oil, vinegar or lemon juice, garlic (if using), and salt.
Shuck the corn and cut off kernels. To help keep the kernels from escaping, cut over a large bowl.
Peel the cucumbers and cut them in half lengthwise. Use a spoon to scrape out and discard the watery seeds in the center. Dice the cucumbers fairly small and add them to bowl.
Core or hull the tomatoes, cutting out the stiff bit at the top where they attached to the vine. Cut the tomatoes in half crosswise (if the tomato were a globe, cut it along the equator). Gently squeeze out any of the watery seeds that come out easily. Dice the tomatoes and add them to the bowl.
Cut off the stem end of the onion and cut it in half lengthwise. Remove and discard the peel. Hold onto the end to keep the onion halves steady as you dice the onion. To mitigate the pungent flavor of the onion, feel free to put the onion in a strainer and rinse it with cool water. Drain completely. Spread the diced onion on layers of paper towels to dry before adding to the bowl.
Toss the vegetables with the dressing well. For an extra burst of flavor, add minced fresh herbs on top before serving.
Serve immediately or store in the fridge. Toss again before serving.