Symmetrical three-phase circuits are fundamental models of power systems. Although the circuits have structural symmetry, asymmetric patterns of oscillations have been observed in real power systems. This paper describes an approach to understanding typical patterns of oscillations in the three-phase circuits using symmetry. In order to figure out oscillation patterns, we introduce a three LC ladder circuit which has a higher symmetry than the three-phase circuit. Using only the symmetries of the three LC ladder circuit, we classify periodic oscillations and construct a lattice of those modes. Further, extending the method to almost periodic oscillations, we decompose and characterize typical almost periodic oscillations by their symmetry. Finally, by observing a global phase space in the three LC ladder circuit, we confirm typical oscillations in the three-phase circuit.