Always thought it would be fun to have a 3some and am curious if I simply ask the provider to arrange the other woman or what is the best way to set this up?

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that you visit a few providers and make sure that you click with them, then pick two that you like and set it up.

Alternatively, take a look at the reviews of two-provider sessions, which you will find by searching reviews for, among many others, Portia Eden, Poppy, Natalie Chambers and Trinity.  You may find that the session described in the review suits you...and off you go!

I'd strongly advise having at least one session with one of the providers first, tho.  You don't want to spend a bundle and discover that someone's just not your cup of tea.

Might be a good idea if the two gals worked together in the past and were compatible with each other as well.