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