Abstract Deep observations have detected extended Lyα emission nebulae surrounding tens of quasars at redshift 2–6. However, the metallicity of such extended halos is still poorly understood. We perform a detailed analysis on a large sample of 80 quasars at z ∼ 3 based on MUSE-VLT data. We find clear evidence of extended emission of the UV nebular lines such as C iv λ1549 or He ii λ1640 for about 20% of the sample, while C iii] λ1909 is only marginally detected in a few objects. By stacking the cubes, we detect emission of C iv, He ii, and C iii] out to a radius of about 45 kpc. C iv and He ii show a radial decline much steeper than Lyα, while C iii] shows a shallower profile similar to Lyα in the inner 45 kpc. We infer that the average metallicity of the circumgalactic gas within the central 30–50 kpc is ∼0.5 solar, or even higher. However, we also find evidence of a component of the Lyα halos, which has much weaker metal emission lines relative to Lyα. We suggest that the high metallicity of the circumgalactic medium within the central 30–50 kpc is associated with chemical pre-enrichment by past quasar-driven outflows and that there is a more extended component of the circumgalactic medium that has much lower metallicity and is likely associated with near-pristine gas accreted from the intergalactic medium. We show that our observational results are in good agreement with the expectations of the FABLE zoom-in cosmological simulations.