In the final act of Sonic 3, Shadow and Ivo Robotnik team up to activate the Eclipse Cannon, a powerful weapon capable of destroying Earth. Shadow, driven by a desire for revenge against GUN for the tragic loss of his friend Maria and haunted by his past, originally aims to use the weapon to obliterate GUN’s headquarters and those responsible. However, Sonic, overcome with anger and desperation after witnessing his friend Tom’s injuries during the chaos, transforms into Super Sonic using the Master Emerald to fight Shadow. The intense battle between Sonic and Shadow pushes them both to their limits, and during this fight, they reach their ultimate powered states—Super Sonic and Super Shadow. As they clash across the world, Sonic succeeds in overpowering Shadow and is about to deliver a fatal blow when Tom’s faith in Sonic’s true goodness helps him resist revenge. Instead of killing Shadow, Sonic offers forgiveness and understanding, recalling his own loss of Longclaw and how love has helped him find hope despite tragedy. This act of mercy causes Shadow to reconsider his actions, and he begins to see things differently, remembering Maria’s kindness and trust in him. Joined by Sonic, Shadow ultimately agrees to help stop Gerald Robotnik’s plan, which involves launching the Eclipse Cannon at Earth. In the climax, Sonic and Shadow work together to divert the weapon's energy, and Ivo Robotnik sacrifices himself along with Shadow to prevent the weapon from destroying the planet. The cannon is ultimately destroyed, narrowly missing Earth and causing a part of the Moon to be sliced off. Sonic, exhausted from the battle, falls from space, prompting Tails and Knuckles to rescue him. Meanwhile, Shadow is shown surviving the destruction and landing somewhere on Earth, hinting at a possible return in future installments. Ivo Robotnik is presumed to have survived as well, setting the stage for more adventures in the Sonic universe. The film ends with a scene teasing new characters, as Sonic's encounter with Amy Rose in a jungle hints at future stories and expanded adventures. Ultimately, the ending emphasizes themes of mercy, hope, and the ongoing battle between good and evil, leaving the door open for more exciting challenges ahead.
