How do we pick her for a thresome???

My husband and I have been married for a year but we are high school sweethearts so we were together on and off for seven years before we got married. Honestly, our sex life hasn't been that great lately but we've been trying to find new things to spice it up. The problem is, my husband's fantacy is to have a thresome but I'm not sure if I want to. The thing is, I'm very paranoid b/c of all of the diseases that are out there. I'd be devastated to know that we were disease-free and then we go out and sleep with some random female and all of a sudden we're itching down there if you know what I mean. So, I guess my question is; should I ignore my husband's wish to have a thresome or if not, how do we go about finding a clean female to do it with? Any advice will be appreciated.