Approaches to level a concrete floor with shallow pipes
Is your idea to level off the center only?  I don't see that working out.  The slope at the outsides would be even worse.  I think you're assuming you normally walk right down the center of the hallway, but I don't think that's true;  if two people pass in the hallway, or you're carrying a bag or something in one hand, you will go down one side or the other.  

Is it possible there's an error in the question?  If the slope is only 3mm per meter wouldn't it have to be a REALLY wide hallway to come to 3cm of rise from the outside to the center?