Communication. You should tell each other what is not preferred in this threesome hookup in a straight and clear way. It is very necessary to have such conversation before having a threesome hookup. It might sound less sexy, but you will be thankful when you are during it. Communicate about the boundaries offers you all an opportunity to reconsider it and it might change someone's mind, but it might be the best. There are some subject I suggest you should talk about: Is it okay for the third to make body contact with each party? Is it okay to kiss one person at a time, or only three persons kissing all together? What threesomes position you want to try? How to deal with penetration?
Know yourself. Get to know yourself in a comprehensive and honest way. Know your boundaries. Believe it or not, many people do not realize their boundaries until they encountered it. For example, you might do not know that you are not okay with your husband kissing the other woman. On the other hand, as the third, are you ready for the role you are going to play? For example, what if you are considered as a sexual toy in their relationship? How are you going to deal with it? If all they want is for you to watch and please yourself? Or if you are only allowed to kiss?
Safe code. It is also necessary for all three some of you to come up with a safe code, which is used when things go out of control or you can't stand. You can't always predict what is going to happen and you will never know. Plus, thinking up a safe code does not mean that you are going to use it. It is just a way for you to feel secure and in control.
You can stop it at any time. You can call it stop in the middle or when everyone is going to be finished. You can stop whenever you feel uncomfortable. Do not think that it might be awkward or it is not a good time to stop. If you think that way, there is never good time to stop. Do not hesitate to stop when you think things are getting awkward or out of control. Believe me, it couldn’t be more sorry that everything is finished with someone feeling regret. As the third, if you think the couple is not ready for a three way experience, do not tempt them into it. It won't end well no matter what.
Make sure you trust each other. Everyone knows that if a couple is going to have a threesome, trust is essential. But little do they know that the third also need to believe that the couple is making a right decision of having a threesome with you. If you think one of them is not stable enough to have a threesome tinder hookup, please decline their invitation with polite. If you don't trust their intention of having a threesome, or you think that they are not going to keep the boundaries, do not do it. It is for the best of the three of you. After all, you are not going to be the sacrifice in their relationship.