Three is harmony. Four is square and so on. By associating numbers to the Trigrams, we enrich both the meaning of numbers and the meaning of the Trigrams. Let us, as did Leibniz in the 16th century, look at the Yin and Yang symbols as binary arithmetic notations. Binary means in two's.
Computers use binary notation.
Arithmetic means counting or numbers. Notation means a way of writing it down. In other words, the Yin and Yang symbols are a way of counting in two's. We normally use a decimal arithmetic notation. The same principle applies in binary arithmetic.