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