In a small saucepan over medium heat, stir together the sugar, cornstarch, salt, mustard, water and vinegar. Bring to a boil and cook, stirring, for 1 minute. Remove from heat and let cool.
In a large bowl, combine the green beans, wax beans, kidney beans, onion and green pepper. Pour the cooled dressing over all and toss to coat. Best if marinated for at least an hour before serving. Yummy!