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We present the first fully 3D MHD simulation for magnetic channeling andconfinement of a radiatively driven, massive-star wind. The specific parametersare chosen to represent the prototypical slowly rotating magnetic O star\theta^1 Ori C, for which centrifugal and other dynamical effects of rotationare negligible. The computed global structure in latitude and radius resemblesthat found in previous 2D simulations, with unimpeded outflow along open fieldlines near the magnetic poles, and a complex equatorial belt of inner windtrapping by closed loops near the stellar surface, giving way to outflow abovethe Alfv\'{e}n radius. In contrast to this previous 2D work, the 3D simulationdescribed 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 withcharacteristic azimuthal separation of $15-20 \degr$. Applying these results ina 3D code for line radiative transfer, we show that emission from theassociated 3D `dynamical magnetosphere' matches well the observed H\alphaemission seen from \theta^1 Ori C, fitting both its dynamic spectrum overrotational phase, as well as the observed level of cycle to cycle stochasticvariation. Comparison with previously developed 2D models for Balmer emissionfrom a dynamical magnetosphere generally confirms that time-averaging over 2Dsnapshots can be a good proxy for the spatial averaging over 3D azimuthal windstructure. Nevertheless, fully 3D simulations will still be needed to model theemission from magnetospheres with non-dipole field components, such assuggested by asymmetric features seen in the H\alpha equivalent-width curve of\theta^1 Ori C.

First 3D MHD simulation of a massive-star magnetosphere with application to H\alpha emission from \theta^1 Ori C