A third stage separator vessel includes a diffuser to promote uniform flow among an array of separation cyclones in the vessel. The vessel has an inlet to receive a gas stream containing solid particles, and the diffuser is mounted at the inlet so that at least some of the stream passes through the diffuser. In an embodiment, a gap between the diffuser and the inlet causes some of the gas stream to flow over an outer surface of the diffuser. The diffuser preferably includes a perforated plate mounted across a wide end of a frustoconical portion. The plate has a plurality of openings through which the flow passes. The openings in the plate have various sizes and positions to direct a greater amount of flow through radially outward portions of the plate than from a central portion of the plate.