The immediate impulsive flow of an incompressible fluid due to a concentrated flux through an otherwise impermeable boundary is investigated analytically in three dimensions. The flow is inviscid and irrotational, and obeys the equipotential condition at the free surface, which is initially horizontal. Various elementary bottom geometries are analyzed: rectangular basins, sloping beaches, semi-cylindrical and hemispherical basins. Special attention is paid to the case of impulsive free-surface flows generated on a uniform sloping beach. A general integral solution is presented and compared against a series solution found for a discrete set of angles. The results are relevant for the modeling of tsunami generation due to rapid bottom deflections.