An interesting article in the April 2013 Journal of Evolutionary Psychology concerning perception of cheating. They found the more intimate a behavior the more likely it is to be perceived as cheating.
The application of this article to threesomes is twofold. First it addresses the underlying behavior that leads to defining an activity as cheating. This can be important for a couple who is exploring the idea and working to understand how they define cheating. Second, it helps with the understanding if a boundary is violated, the type of behavior that may lead to the other feeling as though cheating has occurred.
Finally this article is an academic paper that may not be for everyone. However if you enjoy reading research into sexual practices then this may be something worth reading. To access the article you will need to click on the link and scroll to the bottom of the page then click on the .pdf link for the article.