UBR color selection of LBGs. The selection region (delimited by solid black lines) is calibrated based on the redshift tracks of typical LBG templates (dashed lines). The orange density contours show the LBGs candidates at z ≈ 3.0 − 3.5 that are selected by our criterion. The sources with spectroscopic redshift from MUSE and found within |Δv|≤1000 km s⁻¹ from the central QSO are shown as stars and are color-coded by their visual extinction, A_v, estimated by fitting their SEDs (see Section 3.4). The gray density contours mark all the R-band detection within the FORS2 FoV. Typical errors on the colors are shown in the lower-left corner.