Consider four objects, A, B, and D, It is found that A and B are in thermal equilibrium. It is also found that C and D are in thermal equilibrium. However, A and C are not in thermal equilibrium. One can conclude that

Thermal equilibrium :

When no heat flows between two objects that means both objects are in thermal equilibrium.

The system follows the zeroth law of thermodynamics it means system is thermal equilibrium.

Thermal equilibrium system is defined as when the temperature of the system is uniform.

If A, B and C, D are in thermal equilibrium but A and C are not in thermal equilibrium.