Which of the following are the three main designs of a delayed blowback action?

The three main designs of a delayed blowback action are indeed categorized as Roller, Lever, and Gas. These designs utilize different mechanisms to delay the opening of the action until the pressure has adequately decreased, ensuring reliable cycling and ejection of the spent cartridge casing.

In a roller delayed system, rollers are employed that lock and then release the bolt, allowing for effective control over the speed and timing of the action's opening. The lever delayed blowback design uses a lever mechanism to manage the timing and delay of the bolt’s motion. The gas delayed blowback incorporates gas pressure to create a delay in the bolt's movement, effectively utilizing the pressure created from the fired round to manage the cycling of the action.

This understanding of the delayed blowback system is crucial for firearm operation, maintenance, and troubleshooting, making the identification of these designs essential for anyone studying firearms technology.