  The Upper-Scorpius association (5-11 Myr) contains a unique population of
low-mass (M<30 MJup) brown-dwarfs. The detailed relative characterization of
their physical properties (mass, radius, temperature, composition, ongoing
accretion) offers the opportunity to potentially explore their origin and their
mechanisms of formation. We aim at characterizing the chemical and physical
properties of three young, late-M brown-dwarfs claimed to be companions of the
Upper-Scorpius stars USco161031.9-16191305, HIP77900, and HIP78530 using medium
resolution spectroscopy at UV (R~3300), optical (R~5400), and near-infrared
(R~4300) wavelengths. The spectra of six free-floating analogues from the same
association are analyzed for comparison and to explore the potential physical
differences between these substellar objects. We also aim at analyzing emission
lines at UV and optical wavelengths to investigate the presence of ongoing
accretion processes. The X-Shooter spectrograph at VLT was used to obtain the
spectra of our targets over the 0.3-2.5 $\mu$m range simultaneously. Performing
a forward modelling of the observed spectra with the ForMoSA code (Nested
Sampling Bayesian inference method in using BT-SETTL15 models), we infer the
Teff, log(g), and radius of our objects. Our results are compatible with
physical parameters found for free floating analogues of the association and
with evolutionary-model predictions. However the final accuracy on the Teff
estimates is strongly limited by non-reproducibilities of the BT-SETTL15 models
for late-M brown-dwarfs. We identified emission lines in the spectrum of
several objects attributed to chromospheric activity except for USco1608-2315
for which they are indicative of active accretion. We confirm the x4
over-luminosity of USco161031.9-16191305B. It could be explained by the object
activity and if the companion is an unresolved multiple system.
