Or menage a troi if you prefer. Through some (‘relatively’ benign) conversations the mind’s wandered, and I’m wondering how exactly the etiquette for such a, uh, get-together goes. If there’s a couple involved and you’re one of the couple that seems easy enough where most of your attention goes, but how would one handle the other? Actually, what about the other guy? What’s he supposed to do? Even if everyone was sexually compatible with each other how’s he supposed to add to the night without, uh, stepping on anyone’s toes? (Unless everyone’s into that toe sort of thing) 
Other guy?!? 
The two guys do what they would normally do and the third guy just helps.
Dupedeedoo, here to post the relevant links:
My wife wants to go to a “swing club”. What can I expect?
Oh some of these threads make me nostalgic, am I the only one who would like a Roland update? (yes yes, I know he’s long gone)
Since the OP calls more for opinions rather than a specific factual answer, let’s try IMHO. Moved
samclem GQ moderator
Different etiquette applies for different genders.
I can help with MFF, otherwise I got nothin’.
If it’s a couple and an extra, and you are one half of the couple, don’t completely ignore your partner/spouse/whatever they are in favor of the shiny new person. Check in with your usual partner occasionally, at least, if you’re doing something with the other person that doesn’t require your partner’s immediate participation. Also, don’t hound said new person endlessly afterward for a return bout, and especially don’t get your regular partner to do the hounding for you.
Also, do not assume that just because your partner is bisexual, she even wants a threesome in the first place.
This is something you really have to square away when it happens. It’s hard to decide beforehand. People may change their minds, or get a bit overwhelmed by all the naked people all at once. Also, having two people undress you at once does not necessarily go faster than when just one person does it.
Son, you ain’t confessin’, you’s braggin’!
It doesn’t really matter.
Ham, cheese, tomato.
Cheese, ham,tomato.
Tomato, cheese,ham.
So long as you toast the sandwich it works out fine.
Are you talking about 2 guys and a girl? Cause if that’s the case I can’t help out. If it’s 3 guys I’d have some advice.
You moved it by yourself, samclem? Shouldn’t you have waited for another mod to come along so you could have… er… done it together?
Side note: not that the OP is ever going to return, but what a well-chosen screen name. Bravo.
Assuming that the three are an existing couple plus a guest, the guest should do whatever seems like fun. After all, the guest was invited to add something. If the couple wanted a spectator, they would have said so.
If you’re the guest, you can either ask if your fantasy is a good idea, or you can just start doing something and see if somebody objects. 
Everybody has limits, and it’s not hard to find out what they are.
The etiquette for threesomes is simple. Kate Winslet and Liv Tyler are naked and do whatever I ask them to, and I return the favor.
I’ve had tons of experience with this (prior to my current relationship), and it was in the '60s, '70s and early '80s, before any considerations of “safe sex.” And it was exclusively MMM (or MMMMMM, or MMMMMMMMMMMMMM…).
The only “etiquette” is to be nice to each other and learn to take “no” for an answer. There are things that some people enjoy and other people don’t. Get used to it.
Sometimes there are people who like to just be there and watch. Fine. But if everyone’s participating, even with only three people there are enough body parts to go around; no one has to feel left out. Be creative and learn to share.
What if they ask you to go away and stop bothering them? 
You might as well ask what might happen if the Sun rose in the west, or if George Bush started making wise, well-informed decisions. It ain’t gonna happen.
Besides, as I phrased my answer, I get to speak first. Heheheheh.
Og, no! No updates!!! Please! Why thresh old straw, or beat an old bag of bones?
I’ve got enough disturbed, dysfunctional people who won’t stop talking about it in my professional life.
I think threesome etiquette isn’t that much different than any other social situation. If you’re the host, welcome your guest(s) warmly and allow them to enter ahead of you. Assist any ladies out of their garments before removing your own. Should you decide to dine during the evening, remember: forks go on the right, spoons go on the left, ladles go up your ass.
Afterward, don’t forget to send thank-you notes.
I’ve been involved in an MFM (me in the middle) where it was myself and my husband and a good friend. Two very hetero males, no MM stuff going on. It was all about me. First time for all three of us. We just sort of figured it all out. Lots of beer helps.