As a result of the congestion of current navigation services in the L band, the frequency band between 5010MHz and 5030MHz allocated as C band has returned as a candidate global navigation satellite systems (GNSS). The radio frequency compatibility in C band must be assessed carefully before any new signal or service of GNSS is deployed. Due to the narrow bandwidth, the signals transmitted in C band may be limited. This paper only analyzes the compatibility of the signals as the interoperability or classical signals in L band between BeiDou III (BD III) B1C and GPS L1C/A, L1C, Galileo E1OS in the C band to see whether they are appropriate to transmit in C band. The paper uses degradation of effective carrier-power-to-noise density ratio based on spectral separation coefficient (SSC) as the compatibility assessment methodology. Simulation results reveal the interference between BD III B1C and GPS L1C/A, L1C, Galileo E1OS.