Near-eutectic ternary alloys subjected to thin-sample directional solidification can exhibit stationary periodic growth patterns with an ABAC repeat unit, where A, B and C are the three solid phases in equilibrium with the liquid at the eutectic point. We present an in-situ experimental study of the dynamical features of such patterns in a near-eutectic In-In 2 Bi-Sn alloy. We demonstrate that ABAC patterns have a wide stability range of spacing at given growth rate. We study quantitatively the-di↵usion process that is responsible for the spacing uniformity of steady-state patterns inside the stability interval. The instability processes that determine the limits of this interval are examined. Qualitatively, we show that ternary-eutectic ABAC patterns essentially have the same dynamical features as two-phase binary-eutectic patterns. However, lamella elimination (low-stability limit) occurs before any Eckhaus instability manifests itself. We also report observations of stationary patterns with an [AB] m [AC] n superstructure, where m and n are integers larger than unity.