Stir sugar, cornstarch, salt, mustard, water, and vinegar together in a small saucepan over medium heat. Bring to a boil and cook, stirring, for 1 minute. Remove from heat and let cool.
Combine green beans, wax beans, kidney beans, onion, and green pepper in a large bowl; pour cooled dressing over all and toss to coat. Best if marinated for at least an hour before serving.
Classic Three Bean Salad · Dinner Match Lab | Dinner Match Lab