HK's standard assault rifle and can be described as one of the best 7.62mm NATO battle / assault rifles - reliable, versatile, controllable, non-expensive and, finally, very popular. The G3A3 is a selective-fire automatic weapon that employs a roller-locked delayed blowback operating system. The two-piece bolt assembly consists of a breech (bolt head) and bolt carrier. The bolt is locked in battery by two sliding cylindrical rollers that engage locking recesses in the barrel extension. The breech is unlocked when both rollers are compressed inward against camming surfaces driven by the rearward pressure of the expanding exhaust gases upon the bolt head. As the rollers move inward recoil energy is transferred to the locking piece and bolt carrier which begin to withdraw while the bolt head remains locked in battery. As the bolt carrier clears the rollers the bolt head is unlocked and begins to recoil backwards at a velocity lower than that of the bolt carrier. The bolt also features an anti-bounce mechanism that prevents the bolt from bouncing off the barrel chamber's surface. The spring-powered claw extractor is also contained inside the bolt while the lever ejector is located inside the trigger housing.

The G3 is built using as many stamped parts as possible. The receiver is stamped from sheet steel. The trigger unit housing along with pistol handle frame, also are stamped from steel and hinged to the receiver using the cross-pin in the front of the trigger unit, just behind the magazine housing. Every G3 rifle can be equipped with detachable bipods, claw-type detachable scope mounts. Long-barreled versions can be fitted with bayonet or used to launch rifle grenades from the barrel.

The G3A3 is the most well known version. Drum sights, a fixed plastic buttstock, and a plastic handguard. The handguard came in a slim, ventilated version and a wide version. The latter allows for attachment of a bipod.