Observations of quasar absorption-line systems reveal that the z= 3 intergalactic medium (IGM) is polluted by heavy elements down to H I optical depths τH I≪ 10. What is not yet clear, however, is what fraction of the volume needs to be enriched by metals and whether it suffices to enrich only regions close to galaxies in order to reproduce the observations. We use gas density fields derived from large cosmological simulations, together with synthetic quasar spectra and imposed model metal distributions, to investigate what enrichment patterns can reproduce the observed median optical depth of C IV as a function of τH I. Our models can only satisfy the observational constraints if the z= 3 IGM was primarily enriched by galaxies that reside in low-mass (mtot < 1010 M⊙) haloes that can eject metals out to distances >rsim 102 kpc. Galaxies in more massive haloes cannot possibly account for the observations as they are too rare for their outflows to cover a sufficiently large fraction of the volume. Galaxies need to enrich gas out to distances that are much greater than the virial radii of their host haloes. Assuming the metals to be well mixed on small scales, our modelling requires that the fractions of the simulated volume and baryonic mass that are polluted with metals are, respectively, >10 per cent and >50 per cent in order to match observations.