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