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