The most powerful sources among the blazar family are MeV blazars .
Often detected at z > 2 , they usually display high X- and \gamma -ray luminosities , larger-than-average jet powers and black hole masses \gtrsim 10 ^ { 9 } M _ { \sun } .
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 \gamma -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 _ { \sun } .
The three studied sources have jet power at the level of , or beyond , the accretion luminosity .