There is a photo of Walker flying three at once, so it must be authentic.
Jim Walker came to our town just once, in his Chrysler "woodie" station wagon. He flew a Fireball in a vacant lot, using his U-Reely handle, and launching and recovering the plane himself, without it ever touching the ground! His Fireball was equipped with a 2-speed engine, operated by a 3rd line. but he used only 2 lines that day.