I've gotten an extra man as a "gift" once. We had talked about it for months before (first as a fantasy during sex, then we talked about it calmly as a real-life possibility). We knew each others' boundaries quite well---this would definitely not make a good "Surprise!" gift. We had discussed who we wanted (it wound up being someone who was a fairly close former coworker of his, so I didn't know him at all), and we discussed how we wanted to interact during (condoms for all, and only my boyfriend had anal access), as well as what we'd do after (the third would spend the night at his own house, and my boyfriend and I would spend the rest of the night together cuddling.)
My boyfriend had been apprehensive for a long time, and he was definitely 'doing it for me' before it happened---but it wound up being really wonderful for both of us (and the 'gift' enjoyed it too!). He was surprised how hot it had been, and he didn't feel any jealousy at all (though he had been worried about it). We speculated that the extra effort that I put into showing him that I loved him comforted him a lot. During the threesome, I actually wound up spontaneously doing some things he had always asked about but I'd requested more time to adjust to. We had wanted to do it again but our third moved away shortly after.
Anyway, if you want any more detail, feel free to ask.