IL - Maybe I'm just tired but there was no information in those links. The book, talked ABOUT the concept, cited research, but didn't say anything about it. Of the research cited it mentioned how the author The wiki article was about open marriage, but no one talking about this is talking about an open marriage. I personally think open marriages are a bad thing for long term stability.
The book, which is mostly unviewable without buying cited this...
...a summary discussion of sexal mate-swapping in America,....and weather swinging poses a threat to the institution of marriage in the United States. (1978)) 
It was a book talking about a 1978 paper, and a wiki article on open marriages which have nothing to do with threesomes..
So really, what was one suppose to get out of your post?