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
Unlock all Illustrations
Save 1,000 POWs
Clear the game without using a continue
Play 100 times as Tarma
Play 100 times as Fio
Play 100 times as Eri
Play 100 times as Marco
Reach a score of 1 000 000 points with a friend
Kill 1,000 soldiers
Play with a friend
Clear any mission with no miss
Save 100 POWs
Save Hyakutaro Ichimonji
Kill 500 soldiers
Play with all characters
Turn into a snowman
Clear the game
Obtain items from Madoka Aikawa
Throw 300 grenades
Throw 50 grenades
Reach a score of 500,000 points
Get all weapons
Kill 100 soldiers
Reach a score of 200,000 points
Reach a score of 100,000 points
Save an orangutan
Clear Mission 1
Save 10 POWs
Obtain items from Rumi Aikawa