I can offer my story here, since it's similar. I had threesomes with my ex-gf, when we were still together. The threesomes were not a factor in why we broke up, in fact they might have been one of the better aspects of our relationship.
How it started was she went through my phone text messages (example of why we eventually broke up) and found texts I had exchanged with a guy that I had given a HJ to years back (I've explored, I enjoy touching big cocks, not sucking/fucking them). She started to ask me questions about it so I asked her about her past sexual experiences. Turned out the guy she was with before me had a really big cock, according to her recollection, and that got the subject going. I told her I had a fantasy to see a girl get fucked by a really big cock in person, and we had all the usual conversation points.
1) What if she falls for the guy.
2) What if she's scared or turned off by it.
3) What if my feelings change when it happens.
All discussed, debated, and settled. I wasn't worried about her falling for a guy over his cock, we established that it would end the instant she asked for it if she felt weird, and far from my feelings changing for the worse, my feelings for her increased substantially when she was fucking the other guy.
We actually used (or I should say I used) Craigslist and found a guy close to our age nearby. He was in decent shape, about 9 inches and pretty thick, and we agreed to meet up and chat with him at his place. The first time we went there the conversation started pretty tame, and generic, about as naughty as it was was to ask "so have you done this kind of thing a lot" etc. Eventually I just threw it down and said "So, are we going to the bedroom or what?" We did, and she actually was pretty comfortable or at least willing, and we did it. It was hot, in fact I had to avoid touching myself or I would have cum almost instantly from the sight of the events before me.
We wound up doing it again with this guy, who was a nice intelligent educated guy, and then I found another post from a guy on CL and we wound up doing it with him 3 times.
The second guy was larger than the first guy, we actually measured him at 10 inches with a tailors tape, and very thick and super hard. A bit older, but enormous, and also intelligent and tidy, with his own house and meticulously kept vegetable gardens.
She enjoyed the fucking, we took pics and videos, and even asked from time to time "So how's so and so been lately", which was my signal to shoot them a text and see if they were up for another go. We eventually broke up, for reasons unrelated, and she asked me to delete everything and give her a copy on a flash drive, which I did.
New GF is much sexier, extremely hot, but she's not willing to go into threesome territory. She says once a man fucks her she can't go back, she won't be in love with me anymore. I have argued, discussed, debated, but ultimately accepted that she is sure of her feelings so I've dropped the subject.
Anyway, I was lucky to experience one of my biggest fantasies, and I wish you luck in your pursuit of yours. I would say one thing that I notice is that your wife wanting to talk to the guys to try to be attracted to them seems to be counter-intuitive to me. If it's about a big cock experience, then the kind of attraction you get from talking to someone should be secondary. That's relationship building material, and you're not after a relationship with the guy, you're after his cock. If she waits until she finds someone she has things in common with and could find herself attracted to emotionally, that seems to be a more dangerous proposition than simply the physical big cock fantasy you're actually after.
Good luck, keep us updated, and if you ever come to Austin, TX, I can introduce you to 3-4 guys who have 9+ and experience with threesomes. =]