  This paper introduces ``TOPress3D," a 3D topology optimization MATLAB code
for structures subjected to design-dependent pressure loads. With a primary
focus on pedagogical objectives, the code provides an easy learning experience,
making it a valuable tool and practical gateway for newcomers, students, and
researchers towards this topic. TOPress3D uses Darcy's law with a drainage term
to link the given pressure load to design variables, which is converted to
consistent nodal loads. Compliance minimization subjected to volume constraint
optimization problems with pressure loads are solved. Load sensitivities
arising due to design-dependent nature of the loads are evaluated using the
adjoint-variable approach. The method of moving asymptotes is used to update
the design variables. TOPress3D is constituted by six main parts. Each is
described in detail. The code is also tailored to solve different problems. The
robustness and success of the code are demonstrated while designing a few
pressure load-bearing structures. The code is provided in Appendix B and is
available with extensions in the supplementary material and publicly at
\url{https://github.com/PrabhatIn/TOPress3D}.
