3rd Battalion, 1st Marines (3/1) is an infantry battalion in the United States Marine Corps based out of Camp Horno on Marine Corps Base Camp Pendleton, California. Nicknamed the "Thundering Third", the battalion consists of approximately 1,200 Marines and Sailors and falls under the command of the 1st Marine Regiment and the 1st Marine Division.

The 1st Marine Regiment is an infantry regiment of the United States Marine Corps based at Marine Corps Base Camp Pendleton, California. The regiment is under the command of the 1st Marine Division and the I Marine Expeditionary Force. The 1st Marine Regiment is also sometimes referred to as "Regimental Combat Team 1" or "Inchon".