A fiber laser pumped three-wavelength mid-infrared optical parametric oscillator was demonstrated. The pump source had three wavelengths which were 1 060 nm, 1 065 nm and 1 080 nm and the maximum pump power was 70.7 W. 8.7 W mid-infrared idler output was achieved under the maximum pump power, indicating a 16% slope efficiency. The idler wave had three wavelengths which were 3 132 nm, 3 170 nm and 3 310 nm but the signal wave just had one wavelength locating at 1 604 nm. The experiment result was analyzed in both theory and experiment, and came to a conclusion that in the optical resonator, the optical parametric oscillation happened to the 1 060 nm pump wave and the difference frequency generation happened between the 1 604 nm signal wave and the 1 065 nm, 1 080 nm pump waves separately. The intensity difference between idler waves generated by the difference frequency generation of 1 065 nm and 1 080 nm was also analyzed using the conversion efficiency theory of difference frequency generation.