I've had two threesomes when I was younger. One was fabulous and didn't impact our relationship, one was catastrophic and terminated our relationship within 48 hours. I had no idea how things were going to play out, neither will you. A threesome in a relationship is just like screwing someone in the workplace. It's dangerous, self-destructive, and a sign that something is very wrong with the arrangement. I would never advise either to someone unless you know you're about to quit or be fired.
My personal advice aside, keep this in mind. This isn't like tying up your lover or spanking or playing dress-up. This is not a change to your dynamic in bed, it is a change in the dynamic of your relationship. This spicy little change-up is a living being with their own needs and hang-ups and also vulnerabilities. You could be as destructive to the third in your threesome as you can to your marriage.
If you reach a point where you are determined to try this you need to do the following:
1. Lay the ground rules out as clearly as the English language allows with your spouse, write them down. Establish your comfort zones, who gets to touch who where and establish that the rules will be re-addressed each time threesome occurs, even if it's just to say you're both still good with the rules.
2. Agree that nothing is going to happen until all three parties are completely comfortable with the act and that nobody will be permitted to be criticized for deciding to stop before or during the party.
3. Make plans with your spouse for the end of your marriage. It's not morbid it's just responsible given how likely this is to kill the relationship. If you're mature enough to bring a third person into your bed you're mature enough to discuss a very possible outcome.