Which 1994 game introduced Knuckles to the Sonic series?

Sonic the Hedgehog 3 introduced Knuckles to the Sonic series in 1994. He appears as the guardian of the floating Angel Island and initially acts as an antagonist who obstructs Sonic and Tails.

Dr. Robotnik deceives Knuckles into believing that Sonic is trying to steal the Master Emerald. As a result, Knuckles repeatedly sabotages Sonic’s progress while Robotnik repairs the crashed Death Egg. This makes Knuckles a rival rather than a straightforward villain: he is protecting his island and its emerald from what he believes is a threat.

Sonic 3 was developed by Sega Technical Institute for the Sega Genesis and released in North America and Europe in February 1994. It was designed alongside Sonic & Knuckles, which later gave Knuckles a starring role and his first playable campaign.

A common mix-up is naming Sonic & Knuckles, also released in 1994. That game expanded Knuckles’ role, but Sonic the Hedgehog 3 is his debut in the series.