My DP and I have tried it several times. The best experiences have been some of the best in our lives, and have brought us so much closer together.
Have you had a long chat with the girl? I strongly recommend making sure that she is someone who is genuinely enthusiastic about MFF threesome, and probably would classify herself is bi-curious or bisexual. On some well-known websites, girls will just tick every service. also, if you talk to her in advance, she will know more about what you're expecting and can provide you with what you want. We now have a regular girl who is good friends with us as well, which is a dream scenario for both of us.
Before you tell her what you're expecting, you both need to have an honest conversation about what you really do and really don't want . For example my DP gave the absolute law that I must devote at least 70% of my attention to her not to the girl. I also recommend you have a mutual no quibble quit rule. If either of you decide you want to have some time out from it, or just get dressed and both go together, you just do it. It's much better for your relationship to not get the full value paid for, and to go on making any Negative feelings worse.
And when you do you plan what you want, Think through it practically. You didn't say which birthday it is, that if you've seen some great MFF positions in online porn, say, don't forget that the actors probably have considerably smaller, tummies, and considerably larger ahem attributes than the average middle-aged couple, and they are probably working out several times a day and don't have any replacements, stiff neck's et cetera