Cook the green beans and yellow beans in a large saucepan of salted boiling water for 3 minutes or until tender crisp. Refresh under cold running water. Drain.
Combine the oil and lemon juice in a small bowl.
Place the green beans, yellow beans, cannellini beans, celery and onion in a large bowl. Drizzle over the dressing and season with salt and pepper. Toss until well combined.