The total electron content (TEC) along the raypath is a fundamental control on the quality of computerized ionospheric tomography (CIT) reconstructions. By measuring the frequency shift or phase, a traditional dual-band beacon can retrieve the relative TEC using the differential Doppler technique. However, because of the uncertainty in the phase integral constants, this technique can only approximately retrieve the absolute TEC. In some cases, the TEC errors are very large and significantly degrade the CIT results. A planned Chinese satellite for seismological studies will carry a triband beacon transmitting VHF, UHF, and L band frequencies. Using the differential phase between each pair of frequencies, the phase ambiguity can be resolved and used to determine the phase integral constant, hence greatly improving absolute TEC retrieval. This paper proposes an algorithm for finding the integral constant in the triband beacon technique. The numerical simulations show that electron density images reconstructed from TEC using three frequencies are much better than those reconstructed from TEC using two frequencies. In particular, when the initial electron density used in the iterative scheme is far from the true value, small-scale structures cannot be discriminated using a dual-band beacon but are visible in reconstructions using the three-frequency technique.