A newborn magnetar enclosed in a young supernova is thought to produce a luminous optical transit by tapping most of its spinning-down energy into radiation. Luminosity from the magnetar snowplows its surrounding gas and creates an expanding shell. This shell is subject to fluid instabilities and forms a big density spike that has been found in previous 1D models. Recently, multidimensional simulations suggest that the 1D density spike truncated into fragmentary structures, implying that strong fluid instabilities frequently occur inside magnetar-powered supernovae. We present a high- resolution 3D hydrodynamics simulation in 4pi geometry of a magnetar-powered supernova of a 6 Msun carbon-oxygen star. We find the shell is subject to strong fluid instabilities and that turbulent gas forms inside it. Because an extensive circumstellar medium formed before the star exploded, reverse-shock driven fluid instabilities cause additional mixing from the outer part of the ejecta. These two different mixing mechanisms together break down the spherical symmetry of the supernova ejecta. The resulting mixing of chemical elements are reflected in its spectral observation and polarized features because of the asymmetry of the ejecta.