Ultrashort X-ray pulses from free-electron laser X-ray sources make it feasible 
to conduct small- and wide-angle scattering experiments on biomolecular samples 
in solution at sub-picosecond timescales. During these so-called fluctuation 
scattering experiments, the absence of rotational averaging, typically induced 
by Brownian motion in classic solution-scattering experiments, increases the 
information content of the data. In order to perform shape reconstruction or 
structure refinement from such data, it is essential to compute the theoretical 
profiles from three-dimensional models. Based on the three-dimensional Zernike 
polynomial expansion models, a fast method to compute the theoretical 
fluctuation scattering profiles has been derived. The theoretical profiles have 
been validated against simulated results obtained from 300 000 scattering 
patterns for several representative biomolecular species.