Here is the full list of all 18 Final Fantasy III (3D Remake) achievements. It takes around 200 hours to unlock all of the achievements on Windows.
Defeat the Cloud of Darkness.
Open every treasure chest.
Obtain every weapon in Eureka, the Forbidden Land.
Obtain all the summonable creatures.
Obtain the Ultima Weapon from the Legendary Smith.
Save up 500,000 gil.
Defeat Titan and obtain the Earth Crystal.
Obtain a summonable creature.
Attain level 99 with any job.
Defeat Kraken and obtain the Water Crystal.
Defeat Salamander and obtain the Fire Crystal.
Save up 50,000 gil.
Complete 50% of the Bestiary.
Open 50% of all treasure chests.
Defeat Djinn and obtain the Wind Crystal.
Complete the entire Bestiary.
Defeat Iron Giant.
Attain level 99 with all jobs for all characters.