  We present three-dimensional magnetic power spectra in wavevector space to
investigate anisotropy and scalings of sub-Alfv\'enic solar wind turbulence at
magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft.
The magnetic power distributions are organized in a new coordinate determined
by wavevectors (k) and background magnetic field ($b_0$) in Fourier space. This
study utilizes two approaches to determine wavevectors: singular value
decomposition method and timing analysis. The combination of the two methods
allows an examination of magnetic field properties in terms of mode
compositions without any spatiotemporal hypothesis. Observations show that
fluctuations ($\delta B_{\perp1}$) in the direction perpendicular to k and
$b_0$ prominently cascade perpendicular to $b_0$, and such anisotropy increases
with wavenumber. The reduced power spectra of $\delta B_{\perp1}$ follow
Goldreich-Sridhar scalings: $P(k_\perp)\sim k_\perp^{-5/3}$ and $P(k_{||}) \sim
k_{||}^{-2}$. In contrast, fluctuations within $kb_0$ plane show isotropic
behaviors: perpendicular power distributions are approximately the same as
parallel distributions. The reduced power spectra of fluctuations within $kb_0$
plane follow the scalings: $P(k_\perp)\sim k_\perp^{-3/2}$ and $P(k_{||})\sim
k_{||}^{-3/2}$. Comparing frequency-wavevector spectra with theoretical
dispersion relations of MHD modes, we find that $\delta B_{\perp1}$ are
probably associated with Alfven modes. Moreover, for the Alfv\'enic component,
the ratio of cascading time to the wave period is found to be a factor of a
few, consistent with critical balance in the strong turbulence regime. The
magnetic field fluctuations within $kb_0$ plane more likely originate from fast
modes based on isotropic behaviors.
