Threesomes in Tijuana isn't really that good. For being 'whores' most of these chicas working the 'ZONA' are fairly CONSERVATIVE. They really don't like surprises and it's hard finding a truly bi-chica who will get involved with another chica, like eating pussy, lip kissing, or taking turns sucking your cock.
I had a few threesomes in Tijuana, mostly from SG's because they are so cheap, and most of the time willing do to them. The problem is you do one chica, one at a TIME. You fuck one while the other naked puta -watches and waits her turn. There is a lot of nervous laughing involved so yes, it's lots of FUN, and something different, but the girls won't interact with each other, and you have to do all the WORK.
The BGs aren't that much better, and many will tell you, 'NO' , unless they know the chicas you have in mind, or like others said, let HER choose, and hope for the best as you want good chemistry between the girls and you. The price is usually the the same or $10 to $20 more for each girl, and you will have to pay Double the price for the HOTEL.