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.
Complete the entire Bestiary.
Defeat Iron Giant.
Defeat the Cloud of Darkness.
Open every treasure chest.
Obtain the Ultima Weapon from the Legendary Smith.
Obtain every weapon in Eureka, the Forbidden Land.
Obtain all the summonable creatures.
Save up 500,000 gil.
Attain level 99 with all jobs for all characters.
Defeat Titan and obtain the Earth Crystal.
Open 50% of all treasure chests.
Complete 50% of the Bestiary.
Obtain a summonable creature.
Defeat Kraken and obtain the Water Crystal.
Attain level 99 with any job.
Save up 50,000 gil.
Defeat Salamander and obtain the Fire Crystal.
Defeat Djinn and obtain the Wind Crystal.