The 3rd ID is the main Enclave force in the Midwest during The Supremacy War in the 2290s.
The Third Infantry Division traces its roots all the way back to the Sino-American war. The 3rd ID was the main army Division during the Anchorage Reclamation. It was the backbone of many Amy engagements during the war.
The Third was kept at Navarro when the Chosen One blew up the Rig in 2242. It was transferred east, along with the Majority of the Enclave, and was responsible for mainating the many camps the Enclave had set up around the Capital Wasteland. It was later transferred to Chicago after the Enclave lost Adams AFB, and is the main Infantry force fighting the mutants.