* Perfection and completeness: It was seen as the first complete number, representing beginning, middle, and end. This stems from the idea that 1 is a point, 2 a line, but 3 forms a plane, the simplest complete geometric figure.
* The Holy Trinity: While this is a later Christian association, the concept of a tripartite deity (e.g., the Fates or the Moirai) existed in Greek mythology and philosophy, predating Christianity and influencing its Trinitarian theology. The idea of three powerful entities working together resonated strongly.
* Harmony and Balance: Three often represented balance and proportion, crucial elements in Greek aesthetics and philosophy. Think of the three parts of a tragedy (beginning, middle, end) or the three parts of rhetoric (ethos, pathos, logos).
It's important to note that the symbolic meaning of numbers wasn't rigidly defined in ancient Greece. Their significance often depended on the context and the philosophical or religious system under consideration. While 3 held a prominent place in symbolic thought, it wasn't the only, or necessarily the most important, number with symbolic weight.