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