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We study the transverse clustering properties of high matter density peaks astraced by high column density absorption systems (either Lyman limit systemscharacterized by N(HI)> 2 x 10^{17} cm^{-2} or CIV systems with W_{r}> 0.5 A)at redshifts between 2 and 3 with UVES spectra of two QSO pairs (UM680/UM681 at56 arcsec angular separation and Q2344+1228/Q2343+1232 at 5 arcmin angularseparation) and a QSO triplet (Q2139-4433/Q2139-4434/Q2138-4427 at 1, 7 and 8arcmin angular separation). We find 3 damped Ly-alpha systems (N(HI)> 2 x10^{20} cm^{-2}): 2 coinciding with strong metal systems in the nearby line ofsight and 1 matching the emission redshift of the paired QSO; plus 7 Lymanlimit systems: 4 forming two matching couples and 3 without a correspondingmetal system within ~3000 km/s in the coupled line of sight. In summary, wedetect five out of ten matching systems within 1000 km/s, indicating a highlysignificant overdensity of strong absorption systems over separation lengthsfrom ~1 to 8 h^{-1} Mpc. The observed coincidences could arise in gas due tostarburst-driven superwinds associated with a quasar or a galaxy, or gasbelonging to large scale structures like filaments or sheets. We also determinechemical abundance ratios for three damped Ly-alpha systems. In particular, forthe damped system at z ~ 2.53788 in the spectrum of Q2344+1228, new estimatesof the ratios O/Fe, C/Fe are obtained: [C/Fe]<0.06, [O/Fe]<0.2. They indicatethat O and C are not over-solar in this system.Comment: 14 pages, 15 figures, accepted for publication in A&

High matter density peaks from UVES observations of QSO pairs: Correlation properties and chemical abundances