Gravitational Wave (GW) astronomy provides an independent way to estimate cosmological parameters. The detection of GWs from a coalescing binary allows a direct measurement of its luminosity distance, so these sources are referred to as "standard sirens" in analogy to standard candles. In this talk, I investigate the impact of the Einstein Telescope, a third-generation detector which will detect tens of thousands of binary neutron stars. I focus on the non-flat CDM cosmology and some Dark Energy models. To evaluate the accuracy of cosmological parameters, I consider two types of mock datasets depending on whether or not a short Gamma-Ray Burst is detected and associated with the gravitational wave event using the THESEUS satellite. Depending on the mock dataset, different statistical estimators are applied: one assumes that the redshift is known, and another marginalizes over it, taking a specific prior distribution.