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