Hey dpman, I couldn't comment because I was away on vacation. I would like to say that there is a lot of good advice here in the thread alrready.
I used to never do 3-sums because I thought it was a waste of money. Now a days I like the additional stimulation and I get to see/review more girls. These days, time is of the essence so I always try to do a 3-sum. The 3-sums seem to be getting better and better. Here is my .02$.
1. Make sure the girls are interactive. Indies advertise their duo partners and you can contact them to ask. For the agency girls, read the web site for clues and follow up with the booker. I have asked the booker to ask the escort to select a partner. That worked OK. But do not settle for girls that are non-interactive and not bi-sexual. This is a cardinal rule.
2. You won't need an entire Viagra. Just bite off a little corner. My doctor suggested this and one script last me a couple of years. Too much and I'm rock hard but have trouble with the orgasm. Not a bad thing because I used to go too fast. Experiment a bit. My 3-sums are starting to evolve.
3. Make sure you have a bottle of wine and music and encourage the girls to have fun. Have a few drinks with them as you are flirting and starting the foreplay. It relaxes you and reduces everyone's inhibitions.
4. I always start by kissing both girls and then having them kiss each other. I like to stand and let them do a joint BJ. When we go over to the bed I like DATY and while one provides a BBBJ. I like to get up and grab another glass of wine and I encourage the girls to solo for awhile in a 69. I like to get close to make sure that they are actually 69ing. I will pick the one I want to fuck and finish doggie as she eats pussy.
BONUS: Try this. Last time in Sao Paulo, I had the girls 69 and I fucked the girl on top as they were in the 69. The girl that was licking the other girls clit started licking my nut-sack as I came. After the mind-blowing orgasm we got up and we all gave each other high-fives. After the deed, the Brazileiras looked at each other and I heard something like "eu nao accreditio." It was clear that they got a kick out of it. (see my report in the SP thread Other Cities Escorts.)
Do a search and find the other threads about 3-sum advice. There is a lot of nice details and advice. Read the reviews about 3-sums. I had a nice one in July with Euphoria's Bianka and Chelsey. Bianka was great as my 3-sum foreman. The girls have to be into it. Read, and find out which girls like this. Put them on your TDL. Put Bianka of Euphoria on top of that list. Chelsea was good too but Bianka is a freak and speaks good English.
6. I am an out-call guy so that I can pick a very nice hotel like a Marriott with a King bed and a nice shower and I will have wine, chocolates, music etc and I will answer the door relaxed, clean and dressed up....However, the last five threesomes were at an in-call or Privy/Massage parlor and my all-time favorite was at a pay-by-the-hour love motel so maybe this is less important than I imagined?
Last tip - Do you travel abroad? With the US:Canada exchange rate I figured my 3-sum cost me about 310$ USD. I was in Sao Paulo and my two three sums there cost me 90$ USD each and it was just as good if not better. US prices are out of this world. Save your pennies for a trip to Montreal or better yet Latin America, Germany, or Thailand.
I now have double digit 3-sums under my belt. They are getting better and better. Love em.
. Based on my recent talks with some of the ladies I met on my recent trip, they said it could be lots of fun if everyone is relaxed, get into the mood and the fun will materialize. I do prefer the intimacy of a one on one. A duo is on my bucket list . . . so I'll have to try it at least once, or twice 
