We investigated the magnetic behavior of metal hydrides FeHx, CoHx and NiHx for several concentrations of hydrogen (x) by using Density Functional Theory calculations. Several structural phases of the metallic host: bcc (alpha), fcc (gamma), hcp (epsilon), dhcp (epsilon'), tetragonal structure for FeHx and epsilon-gamma phases for CoHx, were studied. We found that for CoHx and NiHx the magnetic moment (m) decreases regardless the concentration x. However, for FeHx systems, m increases or decreases depending on the variation in x. In order to find a general trend for these changes of m in magnetic metal hydrides, we compare our results with the Slater-Pauling curve for ferromagnetic metallic binary alloys. It is found that the m of metal hydrides made of Fe, Co and Ni fits the shape of the Slater-Pauling curve as a function of x. Our results indicate that there are two main effects that determine the m value due to hydrogenation: an increase of volume causes m to increase, and the addition of an extra electron to the metal always causes it to decrease. We discuss these behaviors in detail. keyword: FeHx, CoHx, NiHx, Ferromagnetic metal hydrides, Slater-Pauling curve, Metal hydrides alloys. (C) 2017 Elsevier B.V. All rights reserved.