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