1. Moving at constant, you are at equilibrium and the restorative force of the scale balances your weight. So the scale exerts a force of 490 N and reads 50.0 kg.
2. The elevator is now moving up and slowing down, so its acceleration is pointing opposite the direction of motion, and Newton's second law says the net force is
F - 490 N = (50.0 kg) (-4.9 m/s²)
where F is the magnitude of the force due to the scale. Solve for F to get
F = 490 N - (50.0 kg) (4.9 m/s²) = 245 N
and the scale would read (245 N)/g = 25 kg. This should make sense; your inertia will cause your body to "want" to continue moving up at the same speed while the elevator slows down, and this relieves some of the upward force needed to keep your body moving with the elevator.
3. With the elevator in free fall, Newton's second law gives
F - 490 N = (50.0 kg) (-9.8 m/s²) ⇒ F = 0 N
and the scale would read 0 kg. Now you're falling with the elevator; the scale doesn't have to counteract your weight anymore.