Here is the full list of all 31 FINAL FANTASY III achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Earned all achievements.
Defeated the Cloud of Darkness and restored peace to the world.
Completed 100% of the Bestiary entries.
Collected treasure from 100% of the chests.
Defeated Xande and entered the world of darkness.
Traveled to all the map locations.
Obtained all weapons sealed in Eureka.
Obtained all hidden items.
Received power from the Earth Crystal.
Reached level 50 for all party members.
Learned all summon magic.
Awoke Unei with Noah's Lute.
Obtained 500,000 gil.
Completed 50% of the Bestiary entries.
Defeated Garuda and saved Saronia.
Collected treasure from 50% of the chests.
Obtained 100,000 gil.
Received power from the Water Crystal.
Received power from the Fire Crystal.
Prevented the destruction of the Tower of Owen.
Defeated 500 monsters.
Completed 10% of the Bestiary entries.
Smashed the boulder and cleared the path.
Obtained 10,000 gil.
Collected treasure from 10% of the chests.
Leveled up one job to level 99.
Defeated 300 monsters.
Leveled up one job to level 50.
Defeated 100 monsters.
Leveled up one job to level 10.
Received power from the Wind Crystal.