Are you sure it was Mach 3.5, I heard it was Mach 2.5?
There was, still is! a transonic tunnel with a 13 feet working section (height if I recall) using using a fantastic wooden propellor, there was a television documentary on it, it's now used/owned by Redbull Racing team to do aerodynamic testing of racing cars.
I think there was a a smaller tunnel which was considerably higher speed, a figure of mach 5 comes to mind, achieved through a reservoir of compressed air I think?
I spent all my time on the airfield site, never went into the tunnel site.