One set of rules that doesn't change are those for the invited party, which I have been a good number of times. Rules I have learned from doing it right from the jump, as well as by me learning from my mistakes. Rules that I've become good enough at that I've bee invited back to play more than once by more than one of the couples I have played with. In fact, one of the couples from my past is one with whom I discovered a good deal of the kinks in my "Getting Out The Kinks" label of "L's X-Ray Vision".
There are 4 top rules, and they are as follows:
Being outnumbered is not normally a criteria by which one should give in to what one sexually desires of you. However, in the case of a threesome to which you were invited to by a couple, the role you play with each of them is not your call. Nor is it your call as to what extent you play that role. It's theirs. However, my credo of "Your body, your call" still stands. So what is your call is how much of that offered limitation you take them up on.
In any case, there is no allowing yourself to do with your body what you don't want just because there's 2 of them, and 1 of you. Notice how I did not say "only 1 of you". Because there being 1 of you does not lessen the respect you should demand of your body. But if they are a well-planning, clear-minded couple, then they already know between the 2 of them how far you can go with each of them, and to respect your limitations. And since they are a couple, you must comply. Why? Because...
2) You Are A Guest
For this reason, if they asked, "Your place, or (one of) ours?", you should always answer to go to their place. Or if they're a couple not living together yet, make the home of one of them be the place to play, not yours.
Since we become more domineering in our homes, as we should be, you hosting a threesome with a couple makes it easier for you to forget your place as a guest to their relationship. For even if you go into it wanting that tryst with that couple to be a one time thing, you should still want to be a good enough playmate to be invited back.
3) Like Both Equally
If your only interactions with a couple are via a website or app, then your attraction is totally based on looks. For words on the screen might be enticing, but they are never confirmation until you meet in person. With such being the case, you should not become the 3 for a threesome if you are into one more than the other. So unless the other party plans on being strictly a voyeur, don't plan to be their 3rd if you only want the "cute" guy of that couple. Such as in an interracial couple, don't hook up with the couple if you only want the white guy, or only his Black boyfriend's supposed "BBC". Or if you want the gym-bodied guy, and not his heavyset significant other. If both members of that couple plan on playing, then have the same level of sexual attraction based on looks for both. It may seem like a strict rule to live by to be part of a threesome, but that's the extra work you have to put in by not having an in-person introduction, as well as to avoid bad karma.
If your introduction to a couple wanting a threesome is an in-person one, then while you should still have just as strong a sexual attraction to both parties, it doesn't have to be a sexual attraction based solely on looks. For sometimes, a guy's nice personality actually is a beauty within that turns into a sexual beauty. I've experienced one-on-ones like this, so it can most definitely happen when meeting a couple. And when it does, make sure it causes just as much of a spark as the spark caused by the partner you consider better looking.
4) Ask Questions
Since this is your body you're giving to these people, you have every right to ask questions of the couple you have the possibility of playing with. In additions to questions regarding to the aforementioned points, you also have a right to ask about each member of the couple's HIV/STI status. Although, if you're playing with condoms, it's a unnecessary question. After all, due to the still present shame about HIV and other STIs, guys can lie. Then there's also the many other cases where with enough sexual encounters and no symptoms to show for it, guys just don't know.
With that said, if the couple seems bothered by you asking those unobtrusive questions, then that's a red flag telling you to remove yourself from that situation. For such a couple is not socially equipped to handle a threesome. In fact, it's most likely that they are one of those couples giving open relationships its negative stigma. For such a couple probably has threesomes because their social ineptitude makes them unable to deal with each other. So inviting you is actually them asking you to be a buffer for the night. A duty that you should never allow yourself to be.
Following these rules should make for a great threesome. For as long as these rules for the unconventional are followed, one has less shame to feel about such a display of the unconventional, and instead reveal in sexual bliss and a feeling of "the more, the merrier" after that threesome.