My question to you is: Do you really want to do this? Just because your friend has done it and liked it, doesn't mean that you will. The only reason to attempt any unconventional sexual activity is its native appeal to you. Unless and until that's the case, pass for now, no matter how fervently your friend reassures you that you'll enjoy it. Threesomes only work if the feelings between the two women are genuinely warm, accepting and supportive. If you prefer your sex intimate and romantic as opposed to casual and adventurous, threesomes may not be for you and nothing anyone says will change that.

Confidence comes from understanding your sexual choices in the first place. Only you can determine these things. I know it seems easier to let your friend take the lead and just "see what happens," but successful experiments depend on good planning, even if you decide to change plans in the middle. Decide ahead of time if there will be sexual contact of any kind between you and your friend, who's going to take the lead if things slow down and who's going to do the negotiating. It's entirely possible to have a great three-way experience without sexual interaction between the women if they simply keep their male partner between them and give him all the attention. If you're not sexually attracted to your friend, this may be the more comfortable approach. It may not jibe with his fantasy of "hot bi-babes" fighting over him, but he'll have plenty of fun either way.