  Using Athena++, we perform 3D Radiation-Hydrodynamic calculations of the
radiative breakout of the shock wave in the outer envelope of a red supergiant
(RSG) which has suffered core collapse and will become a Type IIP supernova.
The intrinsically 3D structure of the fully convective RSG envelope yields key
differences in the brightness and duration of the shock breakout (SBO) from
that predicted in a 1D stellar model. First, the lower-density `halo' of
material outside of the traditional photosphere in 3D models leads to a shock
breakout at lower densities than 1D models. This would prolong the duration of
the shock breakout flash at any given location on the surface to $\approx$1-2
hours. However, we find that the even larger impact is the intrinsically 3D
effect associated with large-scale fluctuations in density that cause the shock
to break out at different radii at different times. This substantially prolongs
the SBO duration to $\approx$3-6 hours and implies a diversity of radiative
temperatures, as different patches across the stellar surface are at different
stages of their radiative breakout and cooling at any given time. These
predicted durations are in better agreement with existing observations of SBO.
The longer durations lower the predicted luminosities by a factor of 3-10
($L_\mathrm{bol}\sim10^{44}\mathrm{erg\ s^{-1}}$), and we derive the new
scalings of brightness and duration with explosion energies and stellar
properties. These intrinsically 3D properties eliminate the possibility of
using observed rise times to measure the stellar radius via light-travel time
effects.
