System A is in thermal equilibrium with B and B is separately in thermal equilibrium with C. Then A and C are in thermal equilibrium, from which law of thermodynamics it follows? 

Here, the $ZerothLawofThermodynamics$ is followed by the three systems A, B and C. Given that system A is in thermal equilibrium with system B and at the same time in equilibrium with C. Hence, they are in thermal equilibrium with each other.