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.
Reach a score of 1 000 000 points with a friend
Get all weapons
Kill 500 soldiers
Save 100 POWs
Play with all characters
Play with a friend
Reach a score of 500,000 points
Throw 300 grenades
Kill 100 soldiers
Reach a score of 200,000 points
Obtain items from Madoka Aikawa
Turn into a snowman
Save an orangutan
Reach a score of 100,000 points
Clear Mission 1
Throw 50 grenades
Save 10 POWs
Obtain items from Rumi Aikawa
Save 1,000 POWs
Clear any mission with no miss
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