  We present a new chemical evolution model for the Galaxy that assumes three
main infall episodes of primordial gas for the formation of halo, thick and
thin disk, respectively. We compare our results with selected data taking into
account NLTE effects. The most important parameters of the model are (i) the
timescale for gas accretion, (ii) the efficiency of star formation and (iii) a
threshold in the gas density for the star formation process, for each Galactic
component. We find that, in order to best fit the features of the solar
neighbourhood, the halo and thick disk must form on short timescales (~0.2 and
~1.25 Gyr, respectively), while a longer timescale is required for the
thin-disk formation. The efficiency of star formation must be maximum (10
Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk
formation. Also the threshold gas density for star formation is suggested to be
different in the three Galactic components. Our main conclusion is that in the
framework of our model an independent episode of accretion of extragalactic
gas, which gives rise to a burst of star formation, is fundamental to explain
the formation of the thick disk. We discuss our results in comparison to
previous studies and in the framework of modern galaxy formation theories.
