Electronic states of an atomic particle situated inside an impenetrable cavity of a simple polyhedral form are studied. The numerical method used here for calculations and its theoretical backgrounds are presented. Numerical estimates for the energy levels of a hydrogen atom inside impenetrable cavities of a tetrahedral, cubic, or more complex shape corresponding to truncated polyhedrons are described. The state ordering of the lowest electronic states for confined hydrogen atom, associated with 1s-3d-states of a free system, is studied with special attention to the role of the vertexes in tetrahedral cavities. Some details of the state ordering for tetrahedral, octahedral, cubic, and some other polyhedral cavities are analyzed.