I know you asked the ladies but as a threesome afficionado myself (ok, who isn't?) I wanted to say something from the client's point of view. Besides, you know me well enough by now to know dispensing unsolicited (and often dubious) advice is one of my favorite pastimes.
Miss Taylor gives good advice as usual. She knows a thing or two (or even three) about having a successful (not to mention fun) menage a trois. I echo her advice to you that I wouldn't rush into finding a doubles partner just yet. As a client I've requested two ladies I've seen before have a doubles with me. Like with everything else - sometimes there is chemistry and sometimes there isn't. You want to find someone who is a good match for you - believe me, it's important. A lackluster (disconnected, disinterested, overly competitive, etc.) doubles session can reflect poorly on both ladies. And having a good, reliable, sane, ethical, fun, sexy doubles partner is a HUGE addition to your provider resume.
I haven't had a bad threesome yet, but I have had a few legendary ones out of - well, let's just say many.

And every one of those unforgettable ones I felt like I was watching something amazing unfold - not only between the girls and I, but between the two girls as well. That's unreal.
Having two ladies that are highly compatible and, preferably (for me anyway) really into each other can turn a nice time into an amazing one.