For me, personally, threesomes ROCK!! As far as advice goes, here's my version:
1) Go right ahead as long as it's what YOU want. If you're only doing it to make your spouse/s. o. happy, then you may want to reconsider.
2) Don't be afraid to communicate!! I've found that open lines of communication not only help keep your primary relationship healthy and intact they also make the threesome experience much more rewarding.
3) Have all parties agree on a set of ground rules. Example: If a female joins you and your man, Is it ok with you if he kisses her? Are you comfortable with him ejaculating inside her? Basically, you should take time to think of every concievable aspect that might bother you and communicate your concerns/feelings to your s. o.
4) As far as it being a 3on3 vs. a 2on1, I must politely disagree with kitkate. My s. o. and I have had several threesomes with both male and female partners and there is no way on Gods green earth I would ever engage with another male. But, watching her engage both he and I at the same time really turns me on. I guess it appeals to my dominant side to see her in that situation.
5) Last but not least, if you have a threesome and it doesn't turn out like you'd envisioned, DON'T WORRY ABOUT IT!!! Skin is wash and wear and drip-dry. Accidents happen. Just realize that there is a monsterously huge difference between love and sex.

I wish you the very best of luck in your quest. Have fun!!

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If the cops didn't see it, I didn't do it!

Old age and treachery WILL overcome youth and enthusiasm.