Hmm...my 3PH tandem disc has four gangs. Each gang has a square axle and the disc blades all have square holes. The entire gang turns together.
However, I had an old offset disc whose blades turned independently of each other. When it was taken apart to replace the blades it was discovered that the square axles had round grooves worn in them that allowed the square holes in the blade to just spin on the axle.
With the price of the disc (not knowing the age) you might expect it is well-worn. You might want to pop off the end nut and see if it has a square axle. It may be worn like my offset disc was.
The solution was to weld up the grooves and grind them back to square. Real weldors will cringe at this since the axles were likely hardened steel. But on the farm you sometimes just have to do what works.