World 3-1 is the first level of World 3 in Super Mario Bros. This stage marks the debut of the Hammer Bro. enemy in the series.
The player starts off next to some ? Blocks, with the right-most block containing a power-up, and Koopa Paratroopas jumping toward the player. Across a gap and some pipes (the second of which leads to an underground area with some coins and another power-up, and exits out to the fourth pipe) is a bridge with some Goombas, and above the bridge is a Hidden Block with a 1-Up Mushroom. In the left-most block of the Brick Block formation afterward is a Super Star, that can make getting through the Hammer Bros. up ahead easier. In the second ? Block on the block formation with the Hammer Bros. is a power-up. A Springboard afterward can help the player get over the Brick Blocks ahead, and the right-most block on the top row has a beanstalkthat leads to Coin Heaven. Next is a set of stone stairs with Goombas walking down them, followed by a group of blocks and enemies, with the bottom-right ? Block containing a power-up. In the row of blocks afterward, there is a Coin Block in the second block from the left. Finally, there is a stone pillar followed by some Koopa Troopas walking down the stone stairs before the Goal Pole.