This paper develops the Triadic Principle (TP): a bounded constitutive ontology of physical occurrence. A physical occurrence is constituted by the coupled realization of Physical Content, Temporal Ordering, and Relational Localization — analytically distinguishable, jointly necessary, and sufficient. Constitutively relevant differences individuate distinct occurrences; their ordered succession accounts for physical development without persistence or continuity as constitutive primitives. TP distinguishes physical constitution from scientific representation, treating Event Representation (ER) as evaluative rather than constitutive. Comparison with existing ontological approaches, tested against cases from deuterium formation to the Wheeler–DeWitt equation, clarifies TP’s scope and shows it registering genuine failure rather than redescribing around it.