Three Bean Salad

1 can green beans

1 pound wax beans

1 can kidney beans, drained and rinsed

1 onion, sliced into thin rings

1/4 cup white sugar, or to taste

2/3 cup distilled white vinegar

1/3 cup olive oil

1/2 teaspoon salt

1/2 teaspoon ground black pepper

1/2 teaspoon celery seed