Oth- as a convenience of language, systems are sometimes also said to be a relation of thermal equilibrium if they are not linked so as to be in a relation.
1st - is a version of conservation of energy, adapted for thermodynamics systems. The law of conservation of energy states that the total energy of an isolated system is constant
2th - is an empirically validated postulate of thermodynamics, but it can be understood and explained using the underlying quantum statistical mechanics.
3th - the entropy of a perfect crystal, at absolute zero kelvin, is exactly equal to zero. At zero kelvin the system must be in a state with minimum possible energy