As, my speciality. First, depends on the type of place/experience you want. Free lancers or MPs? High end or low end. I love three somes, espcially, frankly, when I want to be lazy and be pleased. I like to go into some of the lower end places, pick two or three girls, and give very explicit instructions of what I want. 70e aka the Dungeon is a great place for this- not a romantic setting, if that is what you need.
As stated, Zona Blue is a great place. Michelle, curly hair, and often overlooked by those needing a 20yo, is great in 3somes. I have been with her and about 6 different girls over the years. One technique in the MPS, is to choose one girl and ask her which girl she is comfortable with. Laura and Pamela are a tried and three duo at ZP.
With freelancers, it is importnat to do good interviewing. Tell them exactly what you want (i.e. will it include girl on girl), and watch for ANY hesitation. If they hesitate for a moment, move on.
Or, you can come Peru with a bunch of us who are going down soon. Already, I have a three some lined up with too girls that are non pros, but who liked to partake in some orgies down Lima that I was able to attend. One depends double penetration, form two cocks or a C*ck and dildo- I will need to bring my A game with me. The following day I have rung up a three girl orgy with two of my wingmen. All 19, and very, very new to the pro scene....
Sorry, I digressed- I am ready.... Regardless, rule of thumb is to ask for what you want, directly but with tact (although, it is hard to be tactful about asking two girls if they will lick you ass at the same time

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Does it look like I need this trip?