A three way can make a relationship better and it can also lead to deterioration depending on how a person or a couple wind into it. If you do not have a conversation about it first, it may cause trust issues and self-esteem issues. Talk about it in with your partner with a healthy, communicative and honest way. A threesome can strengthen your relationship if you two are 100% on board and go in as a team.
Find out why you want a threesome. Some people expect it with health reasons. Just like mentioned above. A threesome can make your fantasy of having more than one sex partner to come true and it can explore your sexuality. While there are a few that harbors some unhealthy reasons for it. Some people turn threesome into a kind of manipulation of their partner. They want to prove that they are not good enough or to prove they are quite a catch during the sex. Some people are turned on when watching their partner feel jealousy or just fulfill their infidelity fantasy of cheating on their partner.
Once you find out the reasons why you want a threesome, how do you get your partner on board if he/she is not interested. In fact, threesome is not as complicated and obscene than you think. Just imagine it as watching two people kiss and cuddle. If your partner said "no", it is not the end. He/she may not know the fun and joy it can bring. Make he/she know it.
Who do you want to join you? Do you have a target person? Is he/she someone you know? Do you prefer a stranger or someone you familiar with? Experts believe that it will be better to ask someone you never know to join the threesome. After all, it will be a lot easier if the process does not go well. It will do little influence to your daily life. More specifically speaking, if you want to keep the experience a secret, a stranger who does not share the same social circle with you would be a perfect choice. Besides, there will be less awkwardness if you two do not know each other. But do make sure he/she is free from any contagious illness.
Threesome can be addictive if the experience pleases each party. Therefore, you and your partner will need to decide whether or not the experience was what you wanted. If one person isn't 100% on board, you will probably need to find other ways to spice up your relationship. If it is, you should be lucky to find someone that shares the same interest with you.