In geomechanical modeling, it is a central task to predict the distribution of rock porosity throughout the burial history of a given sedimentary basin. In the upper layers, the evolution of porosity is mainly driven by the mechanical compaction resulting from sediment deposition on the top of the basin and from horizontal tectonic loading; indeed, compaction is three-dimensional. Nevertheless, for simplicity, it is classical in basin simulators to assume oedometric conditions, that is, to neglect the horizontal effects, and thus to describe compaction only as a one-dimensional phenomenon by relying on a vertical porosity-stress law. In this paper, we introduce a model to handle three-dimensional compaction whose core idea is to deduce porosity from a modified vertical porosity-stress law in which horizontal strains are inserted. This gives rise to a simplified model in the sense that, as opposed to a full-dimensional model, it does not require to solve three-dimensional constitutive equations. Finally, we validate our model on a geometrically and lithologically complex test case by comparing our results with those obtained on the same case using a three-dimensional finite-element simulator. We conclude that our model offers a significant gain in computing time and reaches reasonable accuracy, and thus provides a useful tool to users who might need a first, quick insight into the results before engaging in longer and more accurate simulations.