Hello123 - I just recently bought a new, but older model, Knight disc rifle. It was only built to handle two pellets. There was a kit, though, that I also bought that allowed me to go up to three. The kit had a completely new bolt. The main visible difference was the size of the nipple the cap goes on. On the two-pellet bolt, it was tiny. The new bolt is much larger and takes shotgun primers. If I were you, I'd take it by a Knight dealer near you and let them tell you before you try to shoot it with 3 pellets.
Best,
Jon