The flow around and heat transfer from prisms arranged in groups are frequently seen in engineering applications, although relevant studies are mostly limited to simple geometries with two prisms in tandem, side-by-side or staggered arrangements. In this study, a system of nine square prisms in a 3 × 3 arrangement is numerically investigated for prism spacing-to-width ratio L/D = 1.2 − 7.0 at a Reynolds number of 150. The focus is given on investigating the effect of L/D on flow structure, fluid forces, heat transfer, vortex shedding, and recirculation bubbles, where only the center prism is heated. The increase in L/D from 1.2 to 7.0 leads to the formation of five types of flows, including single bluff body flow (L/D < 1.6), reattachment flow (1.6 < L/D < 3.3), lateral-interaction-induced coshedding flow (3.3 < L/D < 4.1), mixed flow (4.1 < L/D < 4.6) and free coshedding flow (4.6 < L/D < 7.0). The reattachment flow corresponds to small fluid forces while the lateralinteraction-induced coshedding flow induces large fluid forces. The maximum heat transfer from the center prism occurs for the lateral-interaction-induced coshedding flow where no recirculation bubble forming on the front or rear surface of the prism. The gap recirculation bubble plays a predominant role in the heat transfer from the front surface. The coherence between heat transfer and flow patterns, including the impacts of shear layer reattachment, flow recirculation, and vortex shedding on heat transfer is imparted.