By optical emission spectrum, we report on the first investigation on the spectral characteristics of filaments in three layers gas gap in dielectric barrier discharge, which filled with gas-mixture of argon and air. The filaments are generated in 1 mm gas gap, 4 mm gas gap and 2 mm gas gap, respectively. With previous single layers gas gap or double layers gas gap of the filaments are very different in terms of spectral characteristics. The emission spectra of the N2 second positive band （C3Πu→B3Πg)are measured, from which the molecule vibrational temperature of the filaments which generated in different gas gap are calculated. Based on the relative intensity of the line at 3914 nm and the N2 line at 3941 nm, the electron average energy of the filaments which generated in different gas gap are investigated. Increasing the content of argon, the change of the molecule vibrational temperature and the electron average energy of the filaments are investigated. It is found the ascending order of the molecule vibrational temperature in the same argon content is: 2 mm gas gap, 1 mm gas gap and 4 mm gas gap. However the ascending order of the electron average energy in the same argon content is: 4 mm gas gap, 2 mm gas gap and 1 mm gas gap. The molecule vibrational temperature and the electron average energy of the filaments decrease with the argon content increasing.