Two-dimensional Riemann problems for zero-pressure gas dynamics with three constant states

The Riemann problems for two-dimensional zero-pressure gas dynamics are solved completely when the initial data take three constant states having discontinuities on x,y-positive and x-negative axes. With the help of characteristic analysis, by studying interactions among delta-shocks, vacuums and contact discontinuities, the Riemann solutions constructed exhibit nine different explicit configurations. The Mach-reflection-like configurations appear in some solutions.