An elevator weigting 400 kg/s is to be lifted up at a constant velovity of 3cm/s.what would be the minimum power of the motor to be used ?

Weight is not measured in kg/s.

I assume you mean the mass is 400 kg.
The weight is then 400 g newtons.
The power required is weight x velocity,
P = 400*9.8*0.03 = 118 watts