Mega Man 3 (1990) took the formula of MM2 and bolted on the pieces fans never let go of: the slide, Rush the transforming robot dog, and the whistling rival Proto Man.

The slide is the important one. Press down and jump and Mega Man shoots along the floor, under a projectile, through a one-tile gap, out of a corner he'd otherwise be trapped in. It gives him a dodge for the first time, and it changes how you read every boss pattern in the game. Every Mega Man made since assumes you have it.

Rush replaces MM2's separate item weapons with one dog in three configurations: Coil for a vertical boost, Marine for underwater sections, Jet for a ride you steer. Folding the traversal tools into a character rather than a menu is a small thing that makes the whole game feel less like inventory management.

Proto Man shows up as Break Man, whistles his four notes, fights you, and leaves. He's the first real character in the series beyond Wily, and the ambiguity about whose side he's on carried the plot for the next three games.

It's also the biggest game in the run to that point. Eight Robot Masters, then four returning stages rebuilt around Doc Robot, a machine that copies the movesets of MM2's bosses and fights you in pairs. Those stages are the hardest thing in the NES series and they are where most playthroughs stall.

Keiji Inafune has said for years that the game shipped unfinished and that the schedule cost them content. You can feel it in places: the difficulty curve lurches, and some stages end abruptly. Most of us were too busy sliding under everything to notice.

Yasuaki Fujita's soundtrack is the other reason this one endures. Snake Man and Spark Man alone would carry a lesser game.