The Typical Massive Quiescent Galaxy at z$\sim$3 is a Post-starburst
  We have obtained spectroscopic confirmation with Hubble Space Telescope
WFC3/G141 of a first sizeable sample of nine quiescent galaxies at 2.4<z<3.3.
Their average near-UV/optical rest-frame spectrum is characterized by low
attenuation (Av$\sim$0.6 mag) and a strong Balmer break, larger than the 4000 A
break, corresponding to a fairly young age of $\sim$300 Myr. This formally
classifies a substantial fraction of classically selected quiescent galaxies at
z $\sim$ 3 as post-starbursts, marking their convergence to the quenching
epoch. The rapid spectral evolution with respect to z $\sim$ 1.5 quiescent
galaxies is not matched by an increase of residual star-formation, as judged
from the weak detection of [O II]3727A emission, pointing to a flattening of
the steep increase in gas fractions previously seen from z $\sim$ 0 to 1.8.
However, radio 3GHz stacked emission implies either much stronger dust-obscured
star formation or substantial further evolution in radio-mode AGN activity with
respect to z $\sim$ 1.5.
