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