XMM-Newton observations of three poor clusters: Similarity in dark matter and entropy profiles down to low mass

(Abridged) We present an analysis of the mass and entropy profiles of three poor clusters (A1991, A2717 and MKW9) observed with XMM-Newton. The clusters have similar temperatures (kT=2.65, 2.53 and 2.58 keV), and similar redshifts (0.04 < z < 0.06). We trace the surface brightness, temperature, entropy and integrated mass profiles up to 0.5 (0.35 for MKW9) of the virial radius (r_200). The integrated mass profiles are very similar in physical units and are reasonably well fitted with the NFW mass model with concentration parameters of c_200=4-6 and M_200=1.2-1.6 X 10^14 h_70^-1 \msun. The entropy profiles are similar at large scale, but there is some scatter in the central region (r<50 kpc). None of the clusters has an isentropic core. Including XMM data on A1983 (kT=2.2 keV), and A1413 (kT = 6.5 keV), we discuss the structural and scaling properties of cluster mass and entropy profiles. The scaled mass profiles display <20% dispersion in the 0.05 - 0.5 r_200 radial range. The c_200 parameters of these clusters, and other values from the literature, are fully consistent with the c_200 - M_200 relation derived from simulations. The dispersion in scaled entropy profiles is small, implying self-similarity down to low mass (kT ~2 keV), and is reduced by 30-40% (to ~20%) if we use the empirical relation S \propto T^0.65 instead of the standard self-similar relation, S \propto T. The mean scaled profile is well fitted by a power law for 0.05 < r_200 < 0.5, with a slope slightly lower than expected from pure shock heating (\alpha = 0.94+/-0.14), and a normalisation at 0.1 r_200 consistent with previous studies. The gas history thus likely depends both on gravitational processes and the interplay between cooling and various galaxy feedback mechanisms.