An integrated on-chip optical device consisting of two distributed feedback (DFB) lasers and one multimode semiconductor ring laser (SRL) has been numerically investigated. In this optical circuit, the two DFB lasers are injected into the SRL, and with the presence of the four-wave mixing effect and optical feedback, the three semiconductor lasers achieve mutual-locking state. The beating between the output optical spectral lines can generate readily tunable radio frequency signals with high spectral purity.