This article presents a numerical study on the behavior of hollow-core fiber-reinforced polymer-concrete-steel (HC-FCS) columns under combined axial compression and lateral loadings. The investigated HC-FCS columns consisted of an outer circular fiber-reinforced polymer (FRP) tube, an inner square steel tube, and a concrete wall between them. The HC-FCS column has several advantages over RC columns. The tubes act a stay-in-place formwork, providing continuous confinement and reinforcement. The concrete shell prevents the outward buckling of the steel tube, which improves the column strength. Three-dimensional (3D) numerical models were developed and validated against experimental results. The models subsequently were used to conduct a parametric finite-element (FE) study investigating the effects of the concrete wall thickness, steel tube width-to-thickness (B/ts ) ratio, confinement ratio, concrete strength, applied axial load level, and buckling instabilities on the behavior of the HC-FCS columns, with a particular emphasis on local buckling of the inner tube. This study revealed that the behavior of HC-FCS columns is complicated due to the interaction of the stiffness of the three different materials: concrete, steel, and FRP. In general, in the HC-FCS columns with square steel tubes, failure was triggered by local buckling of the steel tube followed by FRP rupture. The presence of the concrete wall restrained by the outer FRP and inner steel tubes significantly affected the steel tube buckling. Expressions were also developed to predict the local buckling stresses and strains of the square steel tube.