Marinated Cucumbers, Onions, and Tomatoes

Ingredients:

3 medium cucumbers, peeled and sliced 1/4 inch thick
1 medium onion, sliced and separated into rings
3 medium tomatoes, cut into wedges
1/2 cup vinegar
1/4 cup sugar
1 cup water
2 teaspoons salt
1 teaspoon fresh coarse ground black pepper
1/4 cup oil
1 teaspoon chopped fresh mint (optional)