At first involving a third party into a relationship is more or less trying to destroy what has been built.
You have no idea who is against you and would stop at nothing to destroy you and then you invite this kind of person into your personal relationship, what do you expect?

Communication like I would always advice is one of the best measures to keep a healthy and strong relationship.
Imagine sharing your problems, issues and so on with your partner, it makes you understand your differences, get to know each other better, it helps you avoid situations like, jealousy; because you saw your partner interacting with the opposite sex, lack of attention, neglect, untrustworthiness and what have you!
Nice topic you have there @cool08