Context. Feedback from massive stars plays a crucial role in regulating the growth of young star-forming galaxies (SFGs) and in shaping their interstellar medium (ISM). This feedback contributes to the removal and mixing of metals via galactic outflows and to the clearance of neutral gas, which facilitates the escape of ionizing photons. Aims. Our goal is to study the impact of stellar feedback on the chemical abundances of the ISM in a sample of SFGs with strong emission lines at z ∼ 3. Methods. We selected 35 low-mass SFGs (7.9 < log(M⋆/M⊙) < 10.3) from deep spectroscopic surveys based on their CIII]λ1908 emission. We used new follow-up near-infrared (NIR) observations to examine their rest-optical emission lines and to identify ionized outflow signatures through broad emission line wings detected after Gaussian modeling of [OIII]λλ4959,5007 profiles. We characterized the gas-phase metallicity and carbon-to-oxygen (C/O) abundance of the galaxies using a Te-based method via the OIII]λ1666/[OIII]λ5007 ratio and photoionization models. Results. We find line ratios and rest-frame equivalent widths (EWs) characteristic of high-ionization conditions powered by massive stars. Our sample displays a mean rest-frame EW([OIII]λ5007) of ∼560 Å, while about 15% of the SFGs show EW([OIII]λλ4959,5007) > 1000 Å and EW(CIII]) > 5 Å, closely resembling those now seen in epoch of reionization (EoR) galaxies with the James Webb Space Telescope. We find high Te values, which imply low gas-phase metallicities 12+log(O/H) ∼ 7.5–8.5 (mean of 17% solar) and C/O abundances from 23% to 128% solar, with no apparent increasing trend with metallicity. Our sample follows the mass-metallicity relation at z ∼ 3, with some galaxies showing lower gas-phase metallicities. This results in significant deviations from the fundamental metallicity relation. From our [OIII]λλ4959,5007 line profile modeling, we find that 65% of our sample shows an outflow component, which is found both blue- or redshifted relative to the ionized gas systemic velocity, and the mean maximum velocities are vmax ∼ 280 km s−1. We find a weak correlation between vmax and the star formation rate surface density (ΣSFR) of vmax = (2.41 ± 0.03) × ΣSFR(0.06 ± 0.03). Moreover, we find that the mass-loading factor μ of our galaxy sample is typically lower than in more massive galaxies from the literature, but it is higher than in typical local dwarf galaxies. In the stellar mass range covered by our sample, we find that μ increases with ΣSFR. This suggests that for a given stellar mass, denser starbursts in low-mass galaxies produce stronger outflows. Our results complement the picture drawn by similar studies at lower redshift, suggesting that the removal of ionized gas in low-mass SFGs driven by stellar feedback is regulated by their stellar mass and by the strength and concentration of their star formation, that is, ΣSFR.