Here is a part of the 3rds history.
Commanded then by COL Everette A. Stoutner, the 3rd Brigade established a foothold in the rugged terrain around Pleiku and created a blocking force against Vietcong personnel and supplies infiltrating South along the Ho Chi Minh trail and the Cambodian border. Their primary mission was to destroy any possible crossing sites along the frontier and disrupt any attempts by the North Vietnamese Army to conduct major combat efforts in the Central Highlands.

But I think only the flags moved to Cu chi in August 1967