The Birth: Dr. Bart Hamilton was already an accomplished psychiatrist before Harry Osborn was dumped into his lap for treatment. From that day on, Bart's mind started to get seriously messed up and the monster within started to take control. Treating Harry Osborn for his Green Goblin "delusions," Hamilton began to investigate the claims that Harry made under hypnosis and discovered one of the hide-outs of the original Green Goblin. Drowning in his own delusions of grandeur, he donned the mask and the identity of the Green Goblin in Amazing Spider-Man #176. As Harry's psychoanalyst, Dr. Bart Hamilton made his first appearance officially in Amazing Spider-Man #167 though he may have been treating Harry soon after his initial fight with Spider-Man.

The Legend: The entire life of this third Green Goblin was spread over 5 Spidey issues : Amazing Spider-Man #176 - #180. But Dr. Bart Hamilton meddled in Spidey's affairs long before he actually put on the suit. He planted a hypnotic command in Harry's mind to follow Peter and report his activities to him. After Spidey defeated the second Green Goblin, namely Harry Osborn, Osborn blew the whistle on Spidey's secret identity. Refusing to believe that puny Parker could be Spider-Man or that Harry himself was the Green Goblin, the authorities ushered him to a mental institution where he was treated by Dr. Bart Hamilton. Dr. Hamilton discovered the hidden Goblin lair when he hypnotized Harry during routine part of Harry's treatment. Looking at the equipment he thought that it would be his best opportunity to write definitive thesis on the workings of the criminal mind -- from the criminal's point of view! Problem was that he got caught in the web himself, and soon craved for the power that the Goblin could offer.
He kidnapped Harry Osborn and held him prisoner to keep him out of the way. He then ventured out as the Green Goblin and fooled everyone into thinking that he was the original Green Goblin - incliding Spider-Man; who thought that he was acutally Harry Osborn.
The first sign that the Green Goblin had returned was when the cops suspected that a patient must have done harm to Dr. Hamilton; judging from the way that the good doctor's place was destroyed. Peter rushed to the room he shared with Flash Thompson and Harry Osborn. But the Green Goblin was already there. The Goblin needed a diversion to escape, so he threw Flash out of the window. Peter was suddenly reminded of the way that Gwen Stacy died and he rushed to save Flash in the nick of time.
Though Hamilton was the Green Goblin, he still considered Spidey as his main enemy as he persued the original goals of the Green Goblin. It wasn't like Hamilton was controlling the Goblin suit, it was more likely that the suit was controlling him.
He appeared before Silvermane and his henchmen to take control of the city's crime. The Green Goblin offered to get Silvermane Spidey's head if he gave him the position he craved. A shrewd Silvermane saw it as an opportunity to get rid of both his enemies and agreed to the deal.
Spider-Man had been informed of Aunt May's deteriorating condition and was summoned to sign the surgery release forms. It was during this time that the Green Goblin caught hold of Spidey. Though Spidey appealed to him as Harry Osborn, he refused to let him go. Luckily for him, Silvermane's henchmen opened fire, damaging the goblin glider.
Escaping, he went after Silvermane and threw him from a height to pay him for his treachery. Though Silvermane survived, he was fatally bruised.
It was during the fight that ensued that Spidey pulled off the Goblin mask and found out Gobby's true idenity. And to make matters worse, Harry Osborn escaped from his holding cell and arrived on the scene in his Goblin outfit to duke it out with the doctor. During the struggle Hamilton was killed in an explosion that he himself had caused with his bomb, but luckily, Harry Osborn survived and as usual lost his memory as Gobby.
As for the Green Goblin III's weaponery, he used much the same things that his predessesors had used : the varios gas, smoke and concussion bombs, goblin gliders, razor-edged bats, and a plastic "ghost" he used to engulf and capture Spidey. He had no augmented strength like the previous Goblins.