Here is the full list of all 30 Metal Slug 3 achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Complete all Achievements
Play 100 times as Fio
Play 100 times as Tarma
Unlock all Illustrations
Reach a score of 1 000 000 points with a friend
Save 1,000 POWs
Play 100 times as Eri
Play 100 times as Marco
Kill 1,000 soldiers
Play with all characters
Save Hyakutaro Ichimonji
Kill 500 soldiers
Clear the game without using a continue
Clear the game
Save 100 POWs
Get all weapons
Clear any mission with no miss
Reach a score of 500,000 points
Throw 300 grenades
Kill 100 soldiers
Reach a score of 200,000 points
Obtain items from Madoka Aikawa
Turn into a snowman
Reach a score of 100,000 points
Save an orangutan
Clear Mission 1
Save 10 POWs
Play with a friend
Obtain items from Rumi Aikawa
Throw 50 grenades