It depends on a whole load of things.
You've already said it wouldnt be alien territory for you to have an openmided approach, so no worries there... unless you've only got an open mind, and never done anything 'unusual' before (in which case, you really wont know how its going to make you feel).
You also need to feel totally confident about yourself for the 3 way dynamics to work.
You also need to think about how well you know the 3rd person, and whether it'd be right for you to know them, or if you'd feel better with a 'stranger', the lattewr possibly helping get through any inhibitions you might have. If you know the person well, also think about the fall out if anything goes wrong.
And think about whether this is a one off, or might lead to more, and which of those scenarios you'd prefer.
Speaking from experience, if you've got the right frame of mind and experience, ie you're suited to it, I dont see that it would necessarily end in tears.
Sorry to flow against the tide there