We investigate the trajectories of small particles in a three-dimensional free flow around and through a porous screen. Our model is based on potential flows at different scales coupled to a viscous particle dynamics. We compute numerically the proportion of the particles deviated around a square screen, and thus the section of interception for which particles are impacting or crossing the screen; the three-dimensionality of the flow leads to a particular geometry of this section of interception that can not be deduced from a two-dimensional approach. We further take into account the effect of finite size particles and viscous effects at the pore scale. These results are applied to determine the collection efficiency, i.e the number of particles that impact the fibres. We show in particular how the optimal screen solidity is sensitive to the local Reynolds number based on the pore length scale. Finally, we propose a formula of the collection efficiency of porous screens valid in a wide range of the parameters and discuss the aerodynamic contribution to the collection.