Volcanic eruptions of gassy magmas on Earth, Venus, and Mars produce plumes with markedly different fluid dynamics regimes. In large part the differences are caused by the differing atmospheric pressures and ratios of volcanic vent pressure to atmospheric pressure. For each of these planets, numerical simulations of an eruption of magma containing 4 weight percent gas were run on a workstation. On Venus the simulated eruption of a pressure-balanced plume formed a dense fountain over the vent and continuous pyroclastic flows. On Earth and Mars, simulated pressure-balanced plumes produced ash columns, ash falls, and possible small pyroclastic flows. An overpressured plume, illustrated for Mars, exhibited a complex supersonic velocity structure and internal shocks.