We present 3D hydrodynamic simulations aimed at studying the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies. This evolution is driven by the explosions of type II and type Ia supernovae, whose different contribution is explicitly taken into account in our models. We compare our results with avaiable properties of the Draco galaxy. Despite the huge amount of energy released by SNe explosions, in our model the galaxy is able to retain most of the gas allowing a long period (>3 Gyr) of star formation, consistent with the star formation history derived by observations. The stellar [Fe/H] distribution found in our model matches very well the observed one. The chemical properties of the stars derive from the different temporal evolution between type Ia and type II supernova rate, and from the different mixing of the metals produced by the two types of supernovae. We reproduce successfully the observed [O/Fe]-[Fe/H] diagram.