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