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