We analyze the third harmonic generation by the supercontinuum at 1.5 mum in nonlinear-glass microstructured fibers. The numerical model includes the real field dynamics and allows analyzing the spectral as well as the temporal structure of the generated field. The calculated third harmonic and supercontinuum spectra are compared with the experiment in the SF6 glass PCF pumped by a femtosecond Cr(4+):YAG oscillator. The spectral structure of the third harmonic is composed of the 10-20 high-order modes excited at the phase matching points around 500-550 nm. The individual third harmonic modes have spectral widths of 2-4 nm. In the time domain, the third harmonic signal is a ~10-ps pulse with ps-scale slow modulation, containing subpicosecond ripples at its trailing edge.