  Deep observations are revealing a growing number of young galaxies in the
first billion year of cosmic time. Compared to typical galaxies at later times,
they show more extreme emission-line properties, higher star formation rates,
lower masses, and smaller sizes. However, their faintness precludes studies of
their chemical abundances and ionization conditions, strongly limiting our
understanding of the physics driving early galaxy build-up and metal
enrichment. Here we study a rare population of UV-selected, sub$-L^{*}$(z=3)
galaxies at redshift 2.4$<z<$3.5 that exhibit all the rest-frame properties
expected from primeval galaxies. These low-mass, highly-compact systems are
rapidly-forming galaxies able to double their stellar mass in only few tens
million years. They are characterized by very blue UV spectra with weak
absorption features and bright nebular emission lines, which imply hard
radiation fields from young hot massive stars. Their highly-ionized gas phase
has strongly sub-solar carbon and oxygen abundances, with metallicities more
than a factor of two lower than that found in typical galaxies of similar mass
and star formation rate at $z\lesssim$2.5. These young galaxies reveal an early
and short stage in the assembly of their galactic structures and their chemical
evolution, a vigorous phase which is likely to be dominated by the effects of
gas-rich mergers, accretion of metal-poor gas and strong outflows.
