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