The force acting on the object is the weight of the object, which is given by:
F = m * g
where m = 3.0 kg is the mass of the object and g = 9.81 m/s^2 is the acceleration due to gravity.
F = 3.0 kg * 9.81 m/s^2 = 29.43 N
The net force acting on the object when the lift goes down is the difference between the weight of the object and the force required to accelerate it downward:
Fnet = F - m * a
where a = 0.1 m/s^2 is the acceleration of the lift.
Fnet = 29.43 N - 3.0 kg * 0.1 m/s^2 = 29.13 N
The reading on the spring balance is equal to the net force acting on the object, which is 29.13 N.