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