This article presents a density-based topology optimization method for designing three-dimensional (3D) compliant mechanisms (CMs) and loadbearing structures with design-dependent pressure loading. Instead of interface-tracking techniques, the Darcy law in conjunction with a drainage term is employed to obtain pressure field as a function of the design vector. To ensure continuous transition of pressure loads as the design evolves, the flow coefficient of a finite element (FE) is defined using a smooth Heaviside function. The obtained pressure field is converted into consistent nodal loads using a transformation matrix. The presented approach employs the standard FE formulation and also, allows consistent and computationally inexpensive calculation of load sensitivities using the adjoint-variable method. For CM designs, a multicriteria objective is minimized, whereas minimization of compliance is performed for designing loadbearing structures. Efficacy and robustness of the presented approach is demonstrated by designing various pressure-actuated 3D CMs and structures. © 2021 John Wiley & Sons, Ltd.