After the death of Lawrence Trask, the property of plans for the Sentinels fell to the government of the United States. Dr. Stephen Lang became the head of a federal research about mutants, trying to discover how mutations could provide superhuman powers. However, unbeknownst to his employers, Lang was an anti-mutant bigot who used his position to obtain access to the notes and designs of Larry Trask and Bolivar Trask, his father and creator of the original Mk I Sentinels.
Lang also went to the Council of the Chosen Ones, part of the Hellfire Club, who provided him with funding to create new Sentinels and an orbital HQ. The Council also ignored that Lang wanted to destroy every mutant he could capture.
The Mk III Sentinels were unfortunately inferior to the previous ones because Lang worked with only incomplete notes from the Trasks and because the secret of his activities stopped him from hiring enough experts. He also made them less intelligent than the previous Sentinels to stop any possible rebellion. Notably, Lang built Sentinels that looked like the original X-Men, which became known as the X-Sentinels. The X-Sentinels confused the newer X-Men to such a degree that they were ending up thrashed, but then Wolverine discovered that these impostors were not human and ripped up the Jean Grey lookalike, exposing their true nature.
Lang's Sentinels and space station were destroyed in a battle between the Sentinels and the X-Men. Lang was lobotomized during the battle, but he had made a backup of his mind patterns that was used as a base for the mind of Master Mold and his other Sentinels.
Atmo-Sphere: Mk III Sentinels could generate a life-support bubble to keep up to six human-sized life forms. This allowed them to transport living beings through outer space. The bubble was tough, but not indestructible.
Energy Blast: The Mark III had an amount of different blasts, including but not limited to plasma, electrons, stunners and heat, that could be shot from their palms. They were also armed with a disintegrator that only worked with inorganic matter.
Flight using jets built into their feet.
Mutant Detection: The Sentinels constantly scanned all the living beings in the immediate surrounding area and were able to determine if they were humans or mutants.
Robot: Due to its nature, the sentinels were protected by the sheer strength of the hull. They could not be affected by emotional or mental attacks, or by illusions. However, they were susceptible to attacks affecting machinery.
X-Sentinels: The X-Sentinels were constructed to look, sound, and fight like the original X-Men, including Professor Xavier, Havok and Polaris, even to the point of imitating their originals' powers. Their disguise meant, however, that they might not have been able to employ other standard Sentinel abilities, such as their jet-powered flight.