Wow, this is a difficult question! The emotional conflicts that come with adding a third can be anything from jealousy (obvious) to feelings of inadequecy (not so obvious) and run the gammut in between. Being polyamourous (loving more than one partner) is a difficult lifestyle, though the rewards are great. The difference between poly and swinging is basically the depth of the relationship. Swinging is generally temporary, and does not usually involve a deep, loving relationship. Poly, on the other hand, is a relationship between more than two partners. It is, in a nutshell, loving your partner so much that you get pleasure from seeing them having fun with someone else. In order to get a better definition of this theory, I suggest reading books by Gloria Brame. In order to undertake a swinging or poly relationship, you should both have a very strong relationship to begin with. This is not something to try if your relationship is on the rocks, because those rocks will quickly turn into mountains.

Now, to actually answer your question... I don't think people NEED a third person in a steady relationship. I think it can ENHANCE if the relationship is already strong. Both of you are going to have to judge whether or not your relationship can open up to a third person, and both of you must decide what limits are most important, whether they be strictly monogomous, poly, or swinging.