Have you looked into a modular safes? Snapsafe is a well-known company ttat makes them.
It comes in pieces, but assembles into a "real" safe (residential security containers). The Titan's heaviest piece is the door, and is under 100 pounds. They have lots of options, and folks with houses with tricky turns on stairs, narrow doorways, weak stairwells, or wanting a large save in a basement seem to like this option. For your third floor needs, it may be a good option.
I thought about one, but later found a way to get a safe down stairs by building a turnpoint midway for the size safe I wanted. It cleared by millimeters...