  The study of gas-phase metallicity and its spatial distribution at high
redshift is crucial to understand the processes that shaped the growth and
evolution of galaxies in the early Universe. Here we study the spatially
resolved metallicity in three systems at $z\sim6-8$, namely A2744-YD4,
BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution ($R$ $\sim$
100) spectroscopic observations. These are among the highest-$z$ sources in
which metallicity gradients have been probed so far. Each of these systems
hosts several spatial components in the process of merging within a few
kiloparsecs, identified from the rest-frame UV and optical stellar continuum
and ionised gas emission line maps. The sources have heterogeneous properties,
with stellar masses log($M_*/M_\odot) \sim 7.6-9.3$, star formation rates
(SFRs) $\sim1-15$ $M_\odot$ yr$^{-1}$, and gas-phase metallicities 12+log(O/H)
$\sim 7.7-8.3$, which exhibit a large scatter within each system. Their
properties are generally consistent with those of the highest-redshift samples
to date ($z\sim3-10$), though the sources in A2744-YD4 and COSMOS24108 are at
the high end of the mass-metallicity relation (MZR) defined by the $z\sim3-10$
sources. Moreover, the targets in this work follow the predicted slope of the
MZR at $z\sim 6-8$ from most cosmological simulations. The gas-phase
metallicity gradients are consistent with being flat in the main sources of
each system. Flat metallicity gradients are thought to arise from gas mixing
processes on galaxy scales, such as mergers or galactic outflows and SN winds
driven by intense stellar feedback, which wash out any gradient formed in the
galaxy. The existence of flat gradients at $z\sim6-8$ sets also important
constraints on cosmological simulations, whose predictions on the cosmic
evolution of metallicity gradients often differ significantly, especially at
high redshift.
