  Interstellar bubbles appear to be smaller in observations than expected from
calculations. Instabilities at the shell boundaries create three-dimensional
ef- fects, and are probably responsible for part of this discrepancy. We
investigate instabilities and dynamics in superbubbles by 3D hydrodynamics
simulations with time-resolved energy input from massive stars, including the
supernova explosions. We find that the superbubble shells are accelerated by
supernova explosions, coincident with substantial brightening in soft X-ray
emission. In between the explosions, the superbubbles lose energy efficiently,
approaching the momentum-conserving snowplow limit. This and enhanced radiative
losses due to instabilities reduce the expansion compared to the corresponding
radiative bubbles in pressure-driven snowplow models with constant energy
input. We note generally good agreement with observations of superbubbles and
some open issues. In particular, there are hints that the shell velocities in
the X-ray-bright phases is underpredicted.
