The dynamic critical exponent z is determined from numerical simulations for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models with relaxational dynamics. It is suggested that the dynamics is characterized by two distinct dynamic critical indices z0 and z related to the divergence of the relaxation time τ by τ∝ξz0 and τ∝k-z, where ξ is the correlation length and k the wave vector. The values determined are z0≈1.5 and z≈1 for the 3D LCG and z0≈1.5 and z≈2 for the 3D XY model. Comparisons with other results are discussed.