We present the first fully 3D MHD simulation for magnetic channeling and confinement of a radiatively driven , massive-star wind . The specific parameters are chosen to represent the prototypical slowly rotating magnetic O star \theta ^ { 1 } Ori C , for which centrifugal and other dynamical effects of rotation are negligible . The computed global structure in latitude and radius resembles that found in previous 2D simulations , with unimpeded outflow along open field lines near the magnetic poles , and a complex equatorial belt of inner wind trapping by closed loops near the stellar surface , giving way to outflow above the Alfvén radius . In contrast to this previous 2D work , the 3D simulation described here now also shows how this complex structure fragments in azimuth , forming distinct clumps of closed loop infall within the Alfvén radius , transitioning in the outer wind to radial spokes of enhanced density with characteristic azimuthal separation of 15 - 20 \degr . Applying these results in a 3D code for line radiative transfer , we show that emission from the associated 3D ‘ dynamical magnetosphere ’ matches well the observed H \alpha emission seen from \theta ^ { 1 } Ori C , fitting both its dynamic spectrum over rotational phase , as well as the observed level of cycle to cycle stochastic variation . Comparison with previously developed 2D models for Balmer emission from a dynamical magnetosphere generally confirms that time-averaging over 2D snapshots can be a good proxy for the spatial averaging over 3D azimuthal wind structure . Nevertheless , fully 3D simulations will still be needed to model the emission from magnetospheres with non-dipole field components , such as suggested by asymmetric features seen in the H \alpha equivalent-width curve of \theta ^ { 1 } Ori C .