Evil Robot

Trio, or the evil robot (悪のロボット) is a turncoat alien robot that Duo was fighting with in the beginning of Mega Man 8. He was destroyed by Duo in a battle which resulted in the two robots crashing to Earth. Duo managed to damage him to the point where his body went completely inert, but Trio did enough damage to Duo where neither one could slow down to avoid burning up upon reaching Earth. Though Trio was destroyed, the Evil Energy he harnessed lived on, which is what Duo spent the remainder of the game trying to destroy.

Trio's appearance is almost identical to Duo's original form, albeit mirrored (opposite giant hands to Duo) and with a more "evil" looking armor and color-scheme. His head is also different, with slots under his eyes rather than Duo's T-Visor. Just as Duo has two wings on his back to match the enumeration of his name, Trio has three wings spreading from his back.

Mega Man (Archie Comics)
Trio has a brief appearance in Issue 42, where Shadow Man, when reflecting on his past, remembers an encounter with a robot similar to Duo belonging to a group called the Star Marshals. Along with a robot named Quartet, this robot decimated Shadow Man's fellow Kuiper Droids as they were attempting to escape the battle between the Marshals and the Stardroids and escort their master, Ra Moon to safety. The evil robot ends up saving Shadow Man by stabbing Quartet in the back with his arm, to even Shadow Man's horror. Trio defects from the Star Marshals after seeing the power promised by Ra Moon. He allows Shadow Man to escape and take Ra Moon to Earth while he makes up an excuse to his superiors.

Other appearances
Trio has a short appearance in Mega Man Gigamix and the Rockman 8 manga. Here, he is the combined form of the Stardroids, and after being defeated in battle, Trio splits into the aformentioned robots.

Trivia

 * Before his name was revealed, fans refered to Trio as "Oud" due to his appearance being a mirror of Duo, and some refer to him as "Duo", believing it and Duo to be two halves of a robot. His Archie Comics name of Trio is obviously a play off of Duo's name, as both are indicative of a group of musicians.
 * The evil robot slightly resembles Sigma as he appears Mega Man X3.
 * His appearance in the Archie comics marks the first time in any medium he has been given a name.