By using the exact diagonalization method, a system of three electrons confined in a parabolic quantum dot in zero magnetic field is studied. The ground-state electronic structures and orbital and spin angular momenta transitions as a function of the confined strength are investigated. We find that the confinement may cause accidental degeneracies between levels with different low-lying states and the inversion of the energy values. The present results are useful to understanding the optical properties and internal electron-electron correlations of quantum dot materials.