  Strong gravitational lensing magnifies the flux from distant galaxies,
allowing us to detect emission lines that would otherwise fall below the
detection threshold for medium-resolution spectroscopy. Here we present the
detection of temperature-sensitive oxygen emission lines from three galaxies at
2<z<3.5, which enables us to directly determine the oxygen abundances and
thereby double the number of galaxies at z>2 for which this has been possible.
The three galaxies have ~10% solar oxygen abundances in agreement with strong
emission line diagnostics. Carbon and nitrogen ratios relative to oxygen are
sub-solar as expected for young metal-poor galaxies. Two of the galaxies are
Lya emitters with rest-frame equivalent widths of 20 A and 40 A, respectively,
and their high magnification factors allow us for the first time to gain
insight into the physical characteristics of high-redshift Lya emitters. Using
constraints from the physical properties of the galaxies, we accurately
reproduce their line profiles with radiative transfer models. The models show a
relatively small outflow in agreement with the observed small velocity offsets
between nebular emission and interstellar absorption lines.
