You will get a much better sessions from escorts than you will bar girls and, yes, it costs more.   For escorts, depending on the agency. figure $100 to $150 per hour per chica or $350+ per chica for an 8-hour overnight session.  If you've go the cash it's money well spend.  Nearly always in life you get what you pay for.--and threesomes aren't an exception to the rule.

In a perfect world you should do a short session with a chica first to see if there is chemistry before doing a long session, a threesome, etc.  But if you don't live in a perfect world:

Pick the chica you like best and ask her who she'd what other girls she'd like to be with then pick on of her recommendaitons.  Or ask the agency who works well with the escort you are most interested in seeing and pick on the chica you most want to see likes working with.

The reason is you get a much hotter session when the trio contains chicas who are sexually interested in each other than when you randomly put two chicas together.  

A final word of caution:  many bar girls fake it.  I've had them tell me how they let their hair fall forward covering the "privates" of the other girl and just pretend to "go down" on the other girl.

Hence:
--You'll have better sessions picking escorts over bar girls.
--Pick two girls who like doing threesome together.
--Have the girls tie their hair back so you get a better view of the girl-girl action