I've had tons of MMF threesomes and have attempted 2 MFF threesomes. With me, a girl and another guy, I tend to get way to jealous and insecure, so though it is a massive fantasy of mine, I have decided to just leave that one lie.
The MMF threesomes were all fun. The only time I ever didn't have fun was when the other guy, who wasn't my boyfriend, thought that that one time now meant that I always wanted to hook up with him, which I definitely didn't.
Really, I could go on and on with advice but the only advice that matters is that all parties involved are completely comfortable with it. Back in those days, we had a rule that only boyfriend could penetrate vaginally, though that rule changed over the years. Be sure to set clear and defined boundaries before going into it.
If you don't want your guy eating someone else out, or something about it doesn't make you comfortable, absolutely open your mouth about it. You're already taking a wonderful step in considering the threesome prior to jumping right into it. Get your mind-juices flowing about the threesome so that you can be prepared when you're in it to deal with any situations that may arise.
Absolutely communicate to the best of your ability, if not better. Threesomes have been known to ruin seemingly great relationships. If you're unsure, insecure, jealous or any other negative emotion, speak up about it. Let your partner know that the size of her boobs makes you insecure or the size of his cock scares you. Don't ever stop communicating, especially during the threesome. Practice visual and body cues so that you don't have to use your words. And beat your partner up if they don't communicate with you back. It's absolutely imperative!
In summary, the two main things you should take from this is that a threesome needs to have CONSENT and COMMUNICATION to be anything that anyone is going to enjoy. After that, things just tend to fall into place.