The ground state energies of three-electron quantum dots in a strong magnetic field are calculated by both the Hartree-Fock (HF) and the numerical diagonalization methods. The electron correlation effects on the ground state energies of the dots are investigated by comparing the results from these two approaches. It is shown that the contribution of the correlations to the ground-state energies is almost negligible and the HF approximation is reliable in this case. But the correlation effects on the magmetic fields at which the ground-state momentum quantum number transitions occur are apparent The reason is also analysed.