There's an option for a third best passport for dual-citizens on the best passports page:
http://multinational.io/best_passport http://multinational.io/best_passport
Impressive! You can add the third one to the url manually, e.g. http://multinational.io/best_passport/the%20United%20States+Canada+Australia http://multinational.io/best_passport/the%20United%20States+...