Raised in the summer of 1864 in response to Governor Evans' pleas for a regiment of Indian fighters, the 3rd Colorado Cavalry existed for only 100 days. This was a volunteer regiment authorized by the War Department, although its makeup, training, and equipment was little better than a militia regiment authorized by the governor. The colonel, who took command in October, 1864, was George Shoup. Until his promotion, he was a lieutenant in the 1st Colorado Cavalry. Despite the peacemaking efforts of the various chiefs who attended the Camp Weld Meeting in September, Colonel Chivington, as district commander, was under pressure from the governor and his own commanding officers not to make peace. Rather, he was advised to use the "Bloodless Third" to suppress the Indians before their hundred-day enlistment expired.

Severe blizzards in October and early November 1864 delayed equipping the 3rd Cavalry. Two companies were stationed on the Platte River Trail, to keep that travel and communication line open. Late in November, the remaining ten companies, along with detachments from the 1st Colorado Cavalry, traveled in great secrecy to Fort Lyon, on the Arkansas River near present day Lamar. There they attached another 125 men from the 1st Cavalry, and a section of artillery. Then, in an overnight march through bitter cold, they moved in on the only group of Indians Chivington could find --- Black Kettle's camp on the Big Sandy. At dawn on November 29, 1864, they attacked, killing about 150 Indian men, women and children, losing only ten soldiers.

Now called the "Bloody Third," the regiment returned to Denver in December, and mustered out on December 31, 1864.