Acoustic intensity can be used for different purposes such as sound source localisation, source separation and spatial audio object coding. Three-dimensional measurement of the acoustic intensity requires the design of special microphone arrays. A theoretical analysis and numerical simulations of intensity measurements using open spherical microphone arrays are presented in this paper. The calculation of the acoustic intensity using signals from an open spherical microphone array is presented first. Error metrics are defined to quantify the magnitude and directional errors. Isotropic operating range is defined as the upper frequency for which both errors are within prescribed bounds. Different array topologies are compared using numerical simulations.