A study for the optimisation of in-stream tidal energy converter array layout for a three-dimensional fluid flow field is presented. The study involves design of experiments, computational fluid dynamics simulations, surrogate model construction, and constrained optimisation. Linear Radial Basis Functions (RBF) are used to build surrogate models as a function of four design variables: streamwise, spanwise, vertical and staggered spacings, with the purpose of approximating the capacity factor of an array with a fixed number of Tidal Energy Converters (TECs). Effects of arrays on maximum and minimum stream velocities at specific locations are also assessed. A constrained optimisation mathematical model is formulated considering geometric, economic and environmental aspects. Results are presented in terms of economic revenue for each optimisation model. The method proves to be very time efficient in the evaluation of numerous tidal energy convert array layouts with a view to satisfying optimality criteria.