No redstone three villager iron farm

An iron farm for three villagers and one zombie with no redstone at all. The villagers sit in narrow cells looking down at the zombie, golems spawn on the platform above and drop onto lava over the hoppers. Location matters more than in most designs, because any unlit cave in range gives the golems somewhere else to spawn. Java only.

• Put lava in the four corners instead of one source in the middle. A centre source pushes the golems to the edges and the drops fall between the hoppers and the wall, so you lose iron that already spawned.
• Make the hopper platform 5x5 rather than 3x3 if you want to catch everything. It costs 25 hoppers instead of 9.
• Villagers stop on the end of the bed, lose sight of the zombie and stop spawning golems. Change the blocks above the sight hole to slabs, or put a trapdoor on the block below the zombie.
• If the villagers hug the back wall and stop moving, use workstations in place of the temporary blocks so they come forward during the day.
• Do not use glass in the kill chamber. Golems land on the ledge between the pane and the block below it and the drops burn in the lava.
• Building it underground or near caves needs everything in range lit and spawnproofed. Several builders found their golems were spawning in caves they could not see.