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