Three-dimensional InGaN/GaN nano-and microstructures with high aspect ratios and large active sidewall areas are still of great interest in the field of optoelectronics. However, when grown by metalorganic chemical vapor deposition (MOCVD), their optical performance can be negatively affected by gradients in thickness and peak emission wavelength along their sidewalls, which is still a key obstacle for using such structures in commercial products. In this work, we present a detailed study on the different mechanisms causing this gradient, as well as means to alleviate it. Gas-phase mass transport and surface diffusion are found to be the two main processes governing the shell growth, and the predominance of one process over the other is varying with the geometry of the 3D structures as well as the spacing between them. Consequently, variations in temperature, which mainly affect surface diffusion, will have a stronger impact on structures with small separation between them rather than larger ones. On the other hand, variations in pressure modify gas-phase diffusion, and thus, structures with a large spacing will be more strongly affected. A proper design of the dimensions of 3D architectures as well as the separation between them may improve the gradient along the sidewalls, but a tradeoff with the active area per wafer footprint is inevitable.