A very powerful Metallix model created by Doctor Robotnik some time after he was deposed. Unlike the previous models which were based on Sonic, the Mark 3 was modelled after Knuckles. It proved to be every bit as strong as the original, perhaps stronger, and remained loyal to Robotnik unlike the Brotherhood. Grimer revealed the blueprints for the Knuckles Metallix to Robotnik in Issue 72 in the final panel of The Return of Chaotix, Part 6. The finished product made its first appearance Issue 108.
A New Sonic Terminator
Robotnik built the Mark 3 after establishing an alliance with the Drakon Empire. With his new Sonic-killer ready, Robotnik returned to Mobius and attacked the Freedom Fighters' new base in the Emerald Hill Zone. The new Metallix performed well and subdued Sonic quickly, knocking him out and bringing him back to Drak via Mobius Ring. When the Drakons placed Sonic on trial-by-combat, Metallix was appointed as Prosecutor. The robot proved to be more than a match for Sonic, easily withstanding the hedgehog's attacks and dealing its own out in kind. Although it had the upper hand, Metallix still failed to defeat Sonic. This was only due to the interference of Tails, who had opened the heavy gates to the Arena of Judgement to allow Sonic to escape. When Metallix pursued, Tails pressed the switch to close the gate again and Metallix was crushed beneath it.
Though the original Mark 3 had been destroyed, it had proven its worth against Sonic and so Robotnik began to build several more. He never had the opportunity to use them, however, since his scheme to acquire the power of the Chaos Emeralds had been put into effect and had subsequently ended with the doctor's apparent death. Even though Sonic and the Freedom Fighters later demolished Robotnik's base, another Mark 3 had been assembled by Grimer and later went on a rampage in the Metropolis Zone. Sonic fought the Metallix but once again failed to defeat it; he was knocked unconscious and taken to Grimer, who scanned the hedgehog's mind in order to find out what had happened to Robotnik. After learning what he needed to know, Grimer ordered the Metallix to send Sonic back to Metropolis and leave some fake wreckage there to make it appear that Sonic had won. This wreckage later turned up in a scrapyard, where it was reconstituted and fused with other robot scraps by The Trickster. This scrap-bot was swiftly defeated by Sonic during that incident. No other real Mark 3's appeared again until the Battle for Mobius, when Robotnik returned as a Drakon proxy and launched a massive planetwide invasion. A pair of Mark 3 Metallix attempted to double-team Sonic but ended up colliding with each other. Metallix being hardy as they are, they were probably still functional after that little mishap.
The Mark 3 has proven to be the most effective Metallix model to date, since Sonic has never actually managed to destroy one and has been KOed in two of the three one-on-one fights he's had with them. Being based on Knuckles, the Mark 3 is very strong and has excellent endurance. Like all Metallix, its torso houses a laser cannon. Unlike the other models, it cannot match Sonic's speed and it has never displayed flight or even gliding ability.