  The most powerful sources among the blazar family are MeV blazars. Often
detected at $z>2$, they usually display high X- and \gm-ray luminosities,
larger-than-average jet powers and black hole masses $\gtrsim 10^9 M_{\odot}$.
In the present work we perform a multiwavelength study of three high redshift
blazars: 3FGL J0325.5+2223 ($z=2.06$), 3FGL J0449.0+1121 ($z= 2.15$), and 3FGL
J0453.2$-$2808 ($z=2.56$), analysing quasi simultaneous data from GROND,
\swift-UVOT and XRT, \nustar, and \fermi-LAT. Our main focus is on the hard
X-ray band recently unveiled by \nustar~(3$-$79 keV) where these objects show a
hard spectrum which enables us to constrain the inverse Compton peak and the
jet power. We found that all three targets resemble the most powerful blazars,
with the synchrotron peak located in the sub-millimeter range and the inverse
Compton peak in the MeV range, and therefore belong to the MeV blazar class.
Using a simple one zone leptonic emission model to reproduce the spectral
energy distributions, we conclude that a simple combination of synchrotron and
accretion disk emission reproduces the infrared-optical spectra while the X-ray
to \gm-ray part is well reproduced by the inverse Compton scattering of low
energy photons supplied by the broad line region. The black hole masses for
each of the three sources are calculated to be $\gtrsim 4 \times 10^{8}
M_{\odot}$. The three studied sources have jet power at the level of, or
beyond, the accretion luminosity.
