Tunnelbot

Tunnelbot, called Gakin (ガキーン) in the Japanese strategy guide, is a sub-boss in Sonic the Hedgehog 3 which you confront at the end of Marble Garden Zone Act 1. It is one of the easiest sub-bosses in the game. Just like Sonic 2's Drill Eggman II, this boss burrows through the ceiling and back. Hit it whenever you can. With Knuckles, there is a rotating spiked platform that you can stand on to compensate for Knuckles' lower jump height.

Trivia

When playing as Sonic & Tails, Sonic can stay at the very left of the boss arena while the second player defeats the boss using Tails. When this is done in Sonic 3 alone, it hangs up the game, but it had been fixed in Sonic 3 & Knuckles.