Mission 2a - Finish the first mission without defeating the "Boss" tank in the south-west part of the map.
Mission 2b - Defeat the "Boss" tank in the south-west part of the map and finish the first mission.
Mission 3 - To unlock the last mission you have to get a A rank in all the previous missions (The first,and both 2a and 2b)
Ruhm - Complete two missions (the first and either 2a or 2b) to unlock this weapon in the main game.