American Kitefliers Association

Hi eagleone, thanks for posting here on our AKA site. Don't know if your a kite maker or just a kite pilot. However, there's a site called   kitebuilder.com , it's a pretty active  forum that has a ton of discussion about various dual line kites, their performance and construction. Always a bunch of friendly guy's there who are extremely knowledgeable willing to help. I've seen Ray demonstrate hundreds of times ,yes he's amazing. There's also a fellow from the New England  area who also fly's three. (not quite the experience level of Ray though.)

CLIFF

The site you want to check is Gone With the Wind - http://www.gwtwforum.com/index.php  It's one of the few places left where people dedicated to dual-line flying hang out. 

Charles Stonestreet, a seasoned multiple-dual-line flier from Richmond, VA checks in and posts there from time to time, and I know Charles works with lots of kites that are either still in production or close enough that you can still find them for sale used.

The aforementioned "kiteflier from New England" is Joe "Photo" Perron - he's been doing it for decades and is always a big hit at festivals.

Flying multiple dual-lines simultaneously has never been my thing, but most practitioners I've known have used something large, slow, and stable (plenty of kites like that around that are still in production).  Where they seem to differ most is in how they rig the control lines for 3 or more kites.