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