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