confuse, mix, interpolate
羼 centers on blending things together in a way that creates disorder or confusion: mixing, interpolating, or confusing distinct elements.
The exact historical development of 羼 is uncertain. It visually combines 尸 with a dense cluster of 羊-like elements, but the original semantic and phonetic roles of these components are not clearly established.
Picture a crowded pen (尸) stuffed with three sheep (羊羊羊) all bleating and pushing together until they become a confused, mixed-up flock. That chaotic blending is 羼.
For サン, imagine the three sheep (三, さん) inside the pen: 三 -> サン, and their mixing creates confusion.