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