A sensor for detecting Zn²⁺, Cd²⁺, and Pb²⁺ ions simultaneously based on the square wave anodic stripping response at a bismuth antimony (Bi–Sb) nanocomposite electrode was developed. The electrode was prepared in situ by electrodepositing bismuth and antimony on the surface of a carbon-paste electrode (CPE) while also reducing the analyte metal ions. The structure and performance of the Bi–Sb/CPE electrode were studied using scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry. Operational conditions including the concentration of Sb and Bi, the type of electrolyte, pH, and preconcentration conditions were optimized. The linear ranges were determined to be 5–200 μg L⁻¹ for Zn²⁺, 1–200 μg L⁻¹ for Cd²⁺, and 1–150 μg L⁻¹ for Pb²⁺ with the optimized parameters. The limits of detection were 1.46 μg L⁻¹, 0.27 μg L⁻¹, and 0.29 μg L⁻¹ for Zn²⁺, Cd²⁺, and Pb²⁺, respectively. Furthermore, the Bi–Sb/CPE sensor is capable of selective determination of the target metals in the presence of the common cationic and anionic interfering species (Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺, Mn²⁺, Co²⁺, Cl⁻, SO₄²⁻ and HCO₃⁻). Finally, the sensor was successfully applied to the simultaneous determination of Zn²⁺, Cd²⁺, and Pb²⁺ in a variety of real-world water samples.