Start off with a provider you already know well, and ask her (or she may already list a partner on her site) if she has a partner she likes to do FMF with.

Having good chemistry between the gals is important.

Be in tip-top shape on the day of the session;  it is physically taxing.

Most providers will charge about half as much again as a single gal session, but it varies.

There are lots of threads about two gal sessions on this and other boards.  I would suggest some research on them.

First, be sure that you are comfortable with one of the ladies you are preparing to play with.  That means you will have had rendezvous's with her before.  Second, make sure she is comfortable with the other lady that will be joining you.  It wouldn't hurt if you let the lady you know make suggestions about the third partner.  If you also know her, or of her, that is another plus.   As for TOFTT on this, not recommended.

Besides all of this great advice, this is what you do to make it unforgetable...

Give each lady the same amount of attention.  It's natural for you to probably be a little more attracted to one or the other.  You, and everybody else, will have much more fun if you are constantly thinking of both of the ladies there.  There is only one of you and two of them.  Try to always be in physical contact with each one and switch your position every few minutes so the ladies alternat in being the main focus of your attention.

I do a lot of doubles!  What everybody else says is critical.  if you get all of the basic stuff down, and keep my advice in mind...you're going to have a blast!

To add to what some have said there's basically two subdivisions to this:

1. The 2 Girl Party: this is where the ladies concentrate on you and not each other. This isn't a true threesome. They might lightly fondle each other, kiss, etc. but it will probably resemble how ladies would act if they were gay for pay. Don't get me wrong - I have never been disappointed being with two women. You will have fun and your senses always get overloaded if you have something to wet your dick and for your tongue to lick. This doesn't mean the ladies really aren't into each other, either. They might just be concentrating on giving you pleasure.

2. The Threesome: this is what most think about. However, in practice you might not always get this. This is where the women will go down on each other, focus on giving each other pleasure, and you get to take part.

The dynamics in any M/F/F scenario will always be different. Even if you have seen the ladies individually before and they have seen each other before, but the attitude when you're together will be unique and people will act differently as a result. (this is not a bad thing, but an interesting phenomenon you might not even notice but should be prepared for if you do)

If I may offer a suggestion, decide what you want the dynamic to be and make sure that your first experience is researched. If you have a first lady in mind who you have seen before, and have already started the discussion, consider bringing what I've spoken of here into the discussion. If you don't have the opportunity to discuss, reviews of doubles are you next best bet. Either way, you know what to expect.