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