If she's done it before (and even if she hasn't) I don't think there's a lot of potential for trouble so just lead with "hey, I'd like to have a threesome with you sometime". My personal feelings on the matter from past experience is that it's best if you have already had sessions with both girls. It can be good either way but I think it will be better if there's already been some 1 on 1 with everyone involved. Ask her who she would do a double with and then go see that person. If you get along then you're good to go.

I would let the lady set it up, especially if it's the first time with her.  I only say this because you want both ladies to be comfortable so it's better if they already know or worked with each other.  Also if it is a first with both women, you are essentially asking both ladies for a 3-threesome with 2 unknown people and this can cause unnecessary anxiety and hesitancy due to the "safety" thing.  JMHO.

to make you feel rushed. Personally I go with two hours when someone wants me and another lovely BCD and 3 hours or more if the three of us are going out and about together before the fireworks start!  

Steph xoxo