The Doppler Army was a group of powerful Mavericks in Mega Man X3, residing in Dopple Town and commandeered by Dr. Doppler (although it was later revealed that Sigma was the true mastermind behind the army's riot).
Sometime before the events of Mega Man X3, Dr. Doppler reportedly created an antivirus program that could nullify the Maverick Virus in virtually any Reploid. Following this breakthrough, many of the more advanced Reploids who were cured by Doppler's antivirus joined forces and founded Dopple Town in his honor.
Months later, however, these same Reploids suddenly began to riot, and it was discovered that the antivirus was only a placebo. The newly created Mavericks launched an attack on the Hunter Base in an effort to wipe out the Maverick Hunters, but X and Zero arrived and fended them off. Afterwards, the duo went to Dopple Town to destroy Doppler's Mavericks to prevent them from causing chaos elsewhere.
Dr. Doppler was well aware of X and Zero's efforts, though, so he dispatched the Nightmare Police to deal with the Maverick Hunters. He also discovered and rebuilt Vile, who insisted on pursuing X and Zero as well so he could get his revenge on them. However, all three were defeated along with the rest of the Doppler Army.
Once all of Doppler's Mavericks were destroyed, Dr. Cain used the data from the defeated Reploids to pinpoint the location of Dr. Doppler's secret lab. X and Zero infiltrated the lab and fought Doppler, which ultimately resulted in him coming to his senses. Having been freed from the Maverick Virus, Dr. Doppler revealed that he was being controlled by Sigma the entire time and had been forced to build a gigantic new body for him. X and Zero then went on to confront Sigma and destroy his new body while Doppler worked on a real antivirus program, which was later used on Sigma's final viral form by either Zero or Dr. Doppler, depending on which of the two endings the player achieved. All traces of the Doppler Army were virtually wiped out upon Sigma's defeat.