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