A simultaneous electrochemical determination of serotonin (Ser), melatonin (Mel) and, tryptophan (Trp) was conducted for the first time in the presented research. A metal-metal-metal oxide nanocomposite (CuNi-CeO2-rGO) was synthesized and for modification of glassy carbon electrode (GCE), it was decorated at reduced graphene oxide (rGO). The differential pulse voltammetry (DPV) technique was applied to measurement of Ser, Mel and Trp at the surface of CuNi-CeO2-rGO/GCE. The electrical impedance spectroscopy (EIS) analysis of prepared bare and modified electrodes showed that the CuNi-CeO2-rGO/GCE has the lowest charge transfer resistant in comparison to GCE. The Transmission electron microscopy (TEM) and, X-ray Diffraction (XRD) techniques applied to check the characterization of synthesized nanomaterials. In contrast to the GCE, three separated and well-defined peaks appeared at the CuNi-CeO2-rGO/GCE at 369, 570 and 706 mV for Ser, Mel and Trp in the electrochemical potential window of 0.1-1.0 V. The chemical and electrochemical conditions of analysis were optimized and the detection limit of 5.8 nM (0.0058 µM) for Ser, 6.1 nM (0.0061 µM) for Mel and 6.3 nM (0.0063 µM) for Trp, were calculated based on 3Signal/Noise. The applicability of the CuNi-CeO2-rGO/GCE was investigated by determining target analytes in human urine and blood plasma and comparing the obtained data with HPLC data. The obtained data were in good agreement with each other which demonstrates that the presented method was one of the best analytical methods for the monitoring of Ser, Mel and Trp in the laboratory.