The SPARTAN-III program was a top-secret project initiated by the Beta-5 Division of the Office of Naval Intelligence's Section Three in order to produce cheap and expendable super soldiers to stem the tide of the Covenant's onslaught against the Outer Colonies. In active combat service between 2536 and 2552, the SPARTAN-IIIs turned the tide of several critical battles of the Human-Covenant War. After the war, the remaining SPARTAN-III personnel were given the opportunity to join the newly formed Spartan Operations, supplementing the numbers of the SPARTAN-IVs.
The SPARTAN-III project was both a successor and supplement to the SPARTAN-II program. Created and engineered by Colonel James Ackerson of the Office of Naval Intelligence, the SPARTAN-IIIs were to be the next generation of super soldiers. Composed primarily of vengeful orphans from fallen colonies, they would be cheaper to arm, train, and produce; being chosen from a wider pool of candidates, they would also be more numerous than their SPARTAN-II predecessors. They would rely more heavily on teamwork and sheer numbers than advanced technology and equipment to achieve effectiveness.
The SPARTAN-IIIs were designed to be cheap and expendable. Trained in companies of around 300 at a time with a training regimen tougher than that of the SPARTAN-II program, they were ultimately sent on high-risk missions that the UNSC could not accomplish even with the elite Orbital Drop Shock Troopers. Though the casualty rates of the SPARTAN-IIIs stood at 100 percent on some missions, to ONI, all the operations were strategic successes. They were trading lives for time against the larger and technologically superior Covenant. ONI hoped that in time, enough Spartans would survive to train progressively more future Spartans, swelling the ranks of available super soldiers from only 30 SPARTAN-IIs in 2531 to roughly 100,000 SPARTAN-IIIs within two decades. However, this ultimately never came to pass due to budgetary restrictions and the SPARTAN-III augmentations being compatible with only a small percentage of the population.
There were Three Companies of Spartan-III's.
Alpha Company had 300 Spartan-III's. They were very effective on the missions they were sent on until Operation PROMETHEUS which saw all but 7 Spartan-III's, who were on separate assignments, killed in action. Out of those 7 remaining Alpha Company Spartan-III's, only Jun-A266, Rosenda-A344, Hazel-A302 and Kevin-A282 are still alive.
Beta Company also had 300 Spartan-III's. They were given a more rigorous training than Alpha Company in order to increase their chances of survival on missions. Unfortunately, all but two Beta Company Spartan-III's were killed during Operation TORPEDO. The two surviving Beta Company Spartan-III's were Tom-B292 and Lucy-B091.
Gamma Company is the final company of Spartan-III's trained with a total of 330 Spartan-III's. To further increase their chances of survival on missions they were given three additional drugs during their augmentation process on top of an even more rigorous training. One of these new drugs altered key regions of the frontal lobe to enhance aggression, strength, endurance, and tolerance to injury under stress. This allows the Gamma Company Spartan-III's access to enormous reserves of strength and endurance and enables them to remain fighting even under the influence of a wide systemic shock that would be instantly fatal to a normal human. The other two drugs added were done so to help suppress the negative side effects that can occur due to the altering of the frontal lobe including losing strategic judgment and becoming more prone to impulsive, instinct-driven behavior along with higher levels of paranoia and aggression, even to allies. To completely negate these negative effects, the Gamma Company Spartan-III's must take a cocktail of counter agent's they call "Smoothers" regularly. As of their integration into the Spartan Branch with the other Spartan-II's and Spartan-IV's, their Mjolnir suits automatically administer the "Smoothers". The majority of Spartan-III's are still in operation with a small number having transitioned out of the military and another small number being assigned to more covert roles directly under command of ONI.
Originally, Spartan-III's could not wear the Mjolnir armor that the Spartan-II's wore due to their augmentation process not being as extensive. As a result, they were regulated to wearing Semi-Powered Infiltration armor (SPI Armor). However, with new technologies in the creation of Mjolnir armor with the introduction of the Spartan-IV's, Spartan-III's now utilize Mjolnir.