  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 Alfv\'{e}n 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 Alfv\'{e}n 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.
