What are your thoughts about three way relationships?

I'm in a situation that is new to me and I'm in need of some advice from someone who has experienced something similar.
I'm a straight guy in my mid 20's, I have been with my girlfriend for 3 years now, and I haven't had any experience with threesomes "whether a relationship or a one time thing"
My girlfriend has always expressed her interest in girls and says that she has been with girls before and wants to do it again, so she is Bi or at least bi-curious... Obviously as a guy this was always an erotic idea for me! But so far it has been nothing more than an idea/fantasy.
Anyway, we have recently befriended a lesbian girl and my girlfriend definitely has a huge crush on her and she also seems interested in my girlfriend.
This situation has got me thinking more seriously about my girlfriends interest in other women! This whole issue wouldn't be something I would worry about if it looked like a one time thing but from the amount of texting and calling between them and how often they both want us all to go out together its looking like, at least for my girlfriend, the level of interest is more than a one time thing and there is a possibility where this girl may join our relationship as "my girlfriends girlfriend"
I'm really not a jealous guy and I'm not feeling jealous about this, I'm not bothered about the idea and I don't see it as cheating as long as we are all honest, I'm just not sure if it might backfire.
As for me and our new friend, we get along very well and I couldn't think of a better girl to do this with if I were to do it! Even if my girlfriend loses interest I would most likely still want to be close friends with her so I don't think this would result in me and her fighting which would put my girlfriend in a position to pick sides.
Anyway I had a small talk with my girlfriend and I told her that at the moment I don't mind if they want a threesome, or if they want to mess around on their own, or even a casual relationship, but I wouldn't be comfortable with it unless we are all clear that their relationship is a casual relationship for now, and that I am told if they want to become serious, also that they don't decide to start anything without me knowing and that I could discuss it with the girl to make sure we are all on the same page before anything happens.
Also I do recognize the fact that, since our friend is gay, she may not feel comfortable with me joining them in bed, although I would like to at least watch or be allowed light participation, but I would be ok with keeping our sex lives separate.
To the best of my knowledge, nothing has happened between them and neither has confronted the other weather or not they want to be "more than friends" so nothing has happened yet I just want to make sure I could make a decent decision before anything happens rather than been put on the spot.
So that's my whole situation so far
Has anyone been in a similar situation? How did it go? Do u think its a good idea or its something I should avoid? Do you have any tips/warnings as in do's and dont's? Even if you haven't had experience with a similar situation I would appreciate your thoughts and advice.
And please try to explain your answer, you never know, you might mention something that applies to me but hasn't crossed my mind.
Also I'm sorry if this question isn't relevant to this forum since I'm straight, but all other situations that I could find online involved relationships that had 2 or more Bi members and our scenario would essentially have two members that are just friends and one member involved with both so I wanted to ask for a gay "male or female" point of view and whether or not you would feel comfortable in such a relationship?
Thanks everyone.