  We extend a two-component model for the evolution of fluctuations in the
solar wind plasma so that it is fully three-dimensional (3D) and also coupled
self-consistently to the large-scale magnetohydrodynamic (MHD) equations
describing the background solar wind. The two classes of fluctuations
considered are a high-frequency parallel-propagating wave-like piece and a
low-frequency quasi-two-dimensional component. For both components, the
nonlinear dynamics is dominanted by quasi-perpendicular spectral cascades of
energy. Driving of the fluctuations, by, for example, velocity shear and pickup
ions, is included. Numerical solutions to the new model are obtained using the
Cronos framework, and validated against previous simpler models. Comparing
results from the new model with spacecraft measurements, we find improved
agreement relative to earlier models that employ prescribed background solar
wind fields. Finally, the new results for the wave-like and
quasi-two-dimensional fluctuations are used to calculate ab initio diffusion
mean free paths and drift lengthscales for the transport of cosmic rays in the
turbulent solar wind.
