A 3D multiphase-field (PhF) model has been developed in order to study the formation of a micropore constrained to grow in a solid network (i.e., pinching effect). The model accounts for the pressure difference due to capillarity between liquid and gas, the equilibrium condition at triple (solid-liquid-pore) lines, the partitioning and diffusion of dissolved gases such as hydrogen. From the predicted 3D morphology of the pore, entities such as the Interfacial Shape Distribution (ISD) are plotted and analyzed. It is shown that the mean curvature of the pore-liquid surface, and thus also the pressure inside the pore, is uniform. Despite the complex morphology of pores reconstructed using high-resolution X-ray to mography, the present PhF results suggest that a simple pinching model based on a spherical tip growing in between remaining liquid channels is a fairly good approximation.