Superlensing is an important negative index metamaterial application that extends from modern biomedical fields to high-resolution radar systems for sub-wavelength imaging. In this work, a rigorous simulation study of a 3D polarization independent negative index metamaterial structure has been carried out to investigate its use as a superlens in the microwave region at around 40 GHz. Ten weakly and strongly coupled cross-cylinder fishnet negative index metamaterial layers have been investigated aiming to find a suitable optimized structure to be used as a superlens with a resolution overcoming the half wavelength diffraction limit in the microwave frequency range, especially at around 40 GHz. This study shows that ten weakly coupled layers of optimized cross-cylinder negative index fishnet structure have the advantage of fast convergence of their effective medium parameters. The spacer between consecutive unit cells causes the structure to be mismatched with free space. This results in high reflections and hence it is not suitable to be used as a microwave superlens. When the spacer between consecutive layers is removed the convergence is not as fast as in the previous case but good matching with free space occurs. The figure of merit of the ten strongly coupled cross-cylinder fishnet structure is enormously increased to a peak value of 45 at 39 GHz and the focusing capability of this structure in a double slit configuration shows sub-wavelength resolution.