, you asked, so here's my two cents...
I don't personally know what it's like to be bored in the bedroom, 'cause I always keep it exciting. Edgar and I like to integrate food into our wild bed routine, so this week it's stewed prunes, red hots, and Heinz-57. Throw in some Hamburger Helper and you have a meal and a show! But, I guess that others aren't so lucky. Then again, not everyone has cankles like I do. So, we all have our crosses to bear.
Since you haven't detailed whether you're male or female, or which your partner is, or which any third-party might be, I'm going to call this third person "Lynn". These suggestions would also apply if the person weren't named Lynn. (I imagine that you'd realize that, but I wanted to make sure.) There are several questions that need to be considered before going forward.
The first issue you two have to tackle is: Is Lynn going to have a wangdoodle? If you're a heterosexual or bisexual couple, then this is pretty essential to pin down. (If you're not, then the answer is obvious.) Maybe you're planning on having Mario Lopez join you, but your partner was thinking more of Barbara Walters, and each of you is just assuming that the other person has the same idea.
Second: What should Lynn look like? Short or tall? Thin or plus-sized? Hair or none? Big or small naughty bits? Ethnicity? Age? Number of limbs? And so on. One or both of you may have to compromise. Or heck, you could just choose someone neither of you likes!
Third: What do you two want Lynn to do sexually, and what's not OK? There may be things that the two of you enjoy doing in bed but that are reserved for the two of you only. If you two can agree on the guidelines, then you can communicate these to any prospectives ahead of time. This isn't something that you want to wait to discuss until you're in flagrante delicto.
Fourth: When is check-in and check-out time? Is Lynn arriving, the playtime starting immediately, and then leaving right afterwards? Or, is this going to begin with a social event, like drinks or dinner? Is Lynn allowed to stay the night? You also need to discuss these issues with Lynn before the event begins. Maybe you two are undecided and it's best to say, "We'll see how it goes."
Fifth: What about alcohol or illegal drugs? Will you two be partaking? If so, is it going to affect your judgment and result in you doing something you'll later regret? If not, is it OK if Lynn uses them? Consider that if Lynn is intoxicated and/or high at the end of this event, it's not safe for Lynn to be driving. Unless someone is paying for a cab, you may wind up with an overnight visitor. How does that jibe with your decisions on Issue Four?
Sixth: What are the limits of interaction with Lynn? Is this just someone for sex? Can it become a friendship if you want it to? Is there a rule that any playmate is only invited over once? Is Parcheesi allowed?
Seventh: What if jealousy rears its ugly head? Lynn may have more of a physical attraction to one of you than the other. Or, Lynn may find one of you more capable in the sexual department. Or, Lynn may find one of you more adventuresome. Or, if you and your partner have different types of naughty bits, Lynn may prefer "Tab A" over "Slot B". When one of you feels like the object of desire during this sexual experience and the other feels like a poo patty, then the rejected party is not only going to feel neglected, but is also going to feel inadequate. The plan is that you two are adding a third--not that one of you is being replaced. This hurt can lead to jealousy. There's also the possibility that this three-way sounded terrific before it began, but now one of you simply can't handle it.
Eighth: What are the rules about using "protection"? Mandatory for all involved? Only for Lynn? Throw caution to the wind? Ideally, there's already been some honest disclosure about any sexually-transmitted diseases that each of you has, if any, or any ones that you don't have. But, sometimes people don't provide honest, full disclosure, so everyone needs to protect themselves. And, if unintended pregnancy from this experience is a possibility, any woman or women involved might want to strongly consider using some additional form of birth control even if a man is using a prophylactic; otherwise, it's going to become a substantial problem for all involved down the road.
Ninth: Are you sure that both of you are really on-board with this? Maybe you're all enthused, and your partner begrudgingly agrees but really is against it. Or, it sounds great in theory but then turns out to be a disaster in practice. Or, it's fine for a while and then one of you changes your mind. This whole third-person idea only works so long as you both want to continue. Otherwise, it can drive you two apart.
Tenth: What if Lynn is a dud? One possibility is that you didn't ask for a photo and Lynn resembles something out of a horror movie when you finally meet. Or, there's a photo, but it turns out that it's actually from the time of the Nixon administration. Or, Lynn looks fine but gives off a creepy vibe. Or, the initial impression is great, but once things get started, Lynn has a large wart on the tuckus and wants you to lick it (or something equally unpleasant). Just because this boat has started to leave the dock doesn't mean that you have to take the whole cruise--sometimes bailing is the best idea. You can be forthright--"I don't think that this is going to work"--or you can try some excuse that might or might not be believable: "I just realized that my hair is on fire. We're going to have to cancel."
There are probably other questions, but I'm not getting paid by the word here. I think this should reasonably get you started, though, in sorting through this issue.
And that's the way it is.