Thanks to everybody for your replies. You really opened my eyes to a few things, for instance to the increase in risk. My husband and I have had a number of experiences with three ways and swapping and we always practice safest sex, but you are right that many in the lifestyle do not, and orchestrating the details of safe sex to an uninitiated couple could be a real chore, not to mention the issue of being outnumbered which could also pose a risk.
I definitely appreciate the time you allhave taken to answer my question!