I need some advice/suggestions/recommendations before I can fully agree to a 3some.

Has anyone had a 3some and after their relationship fell apart?
Maybe they wished now that they never did?
Any advice on how I should prepare for doing a 3some?
I want to make sure that we don't know this person, would you recommend an escort business?
I only want to try this once, after trying the 3 some was this a common way to have sex?

any help would be appreciate!

Several questions pop inti my mind very fast. Are you male or female? Sounds like you are trying to set up an ideal situation in 10 min..... It just doesn't happen. Could I suggest going to a swing club a few times get to know some people and then see if they are interested in you and your SO. Then simply get with the program.. I know I've over simplified but that one of the easiest ways to hook up for a 3 some or anything other possible hookup.
Good luck
Ray

Anonymous asks - Has anyone had a 3some and after their relationship fell apart?
Maybe they wished now that they never did?
Any advice on how I should prepare for doing a 3some?
I want to make sure that we don't know this person, would you recommend an escort business?
I only want to try this once, after trying the 3 some was this a common way to have sex?

If you aren't in agreement that this is an experiment then you risk more. You need to talk about this and express your fears ahead of time. Then learn that sex is fun, that being sexual is ok, that its alright to see you lover in someones arms..... it all comes with experiencing it in a positive way.
If you feel pushed, or if this is about someone else's sex life and not your own, you may have some problems.
I have seen relationships break up over threesomes in several different ways. The rarest was that someone in the relationship left the other for the third - I have heard of it happening but not that often. The more common breakup is related to fears, unhealthy relationships (abusive), and general discomfort with exploring sexuality (religious issues, moral issues, whatever).
As far as knowing somebody or not, that is your business. I don't operate that way, it takes warming up and ambiance and such because it is supposed to be fun. You act like it is something that you are stealing in the night, that isn't ok to feel good about, and that you shouldn't be doing. However, its really a choice and you should make that choice carefully so that you can get into it and enjoy it.
As far as three-way sex being common, well, it is amongst swingers..... hee hee hee

have you considered that maybe a threesome is a little ambitious for you? Perhaps 'warming up" to it by going to an on premise club with your spouse or something like that where you can see people swinging and decide for yourself if it is something you want to do would be more appropriate?