Spectral characteristics of three-layered cascaded long-period fiber gratings (LPFGs) were analyzed by the coupled-mode theory together with the transfer matrix method. The effects of the length, position of the fiber that connects the LPFGs on the transmission spectrum of the cascaded LPFG were discussed, and the transmission spectra between the cascaded LPFG and the phase-shifted LPFG were compared. The results show that their spectra have great consistency when the length of the cascading fiber is small or the cascading position is near the end of the cascaded LPFG. But when the cascading fiber is large or the cascading position is in the middle of the cascaded LPFG, the spectra of the cascaded LPFG and the phase-shifted LPFG are different. These results agree well with the two-layered cascaded LPFG. The changes of the film parameters can adjust flexibly the attenuation peak shifts of the spectra and the peak values of the spectra.