Passive galaxies census at z~3

Early-type galaxies (ETGs) are of crucial importance to trace back the galaxy mass assembly across cosmic time yet their formation and quenching remain remarkably elusive. The discoveries of massive, dead galaxies at ever-growing redshifts provided compelling evidence to push their formation up to z$_{f}>$4-5 when the Universe was barely 1 Gyr old. In this talk I will present our results on the ages and Star Formation Histories of a new sample of ETGs at z$\sim3$, built exploiting HST WFC3/G141 rest-frame optical/near-UV grism spectroscopy to study the nature of 10 passive galaxy candidates at 2.5$<$z$<$3.5 in COSMOS.\\ This work is part of a PhD project aimed at quantifying the parent space density of distant \textit{genuine} passive galaxies, which might be underestimated by current cosmological simulations. Therefore, I will also discuss the importance of clarifying the degree of contamination by dusty star-forming galaxies affecting the color selection.