  We present the analysis of the FLAMES dataset targeting the central 25 arcmin
region of the Sextans dSph. This dataset is the third major part of the high
resolution spectroscopic section of the ESO large program 171.B-0588(A)
obtained by the Dwarf galaxy Abundances and Radial-velocities Team (DART). Our
sample is composed of red giant branch stars down to the level of the
horizontal branch in Sextans. It allows to address questions related to both
stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81
stars, which cover the wide [Fe/H]=$-$3.2 to $-$1.5 dex range. The abundances
of 10 other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu.
Despite its small mass, Sextans is a chemically evolved system, with evidence
for the contribution of core-collapse and Type Ia supernovae as well as low
metallicity AGBs. This new FLAMES sample offers a sufficiently large number of
stars with chemical abundances derived at high accuracy to firmly establish the
existence of a plateau in [$\alpha$/Fe] at $\sim 0.4$ dex, followed by a
decrease above [Fe/H]$\sim-2$ dex. This is in stark similarity with the Fornax
and Sculptor dSphs despite their very different masses and star formation
histories. This suggests that these three galaxies had very similar star
formation efficiencies in their early formation phases, probably driven by the
early accretion of smaller galactic fragments, until the UV-background heating
impacted them in different ways. The parallel between the Sculptor and Sextans
dSph is also striking when considering Ba and Eu. Finally, as to the iron-peak
elements, the decline of [Co/Fe] and [Ni/Fe] above [Fe/H]$\sim -2$ implies that
the production yields of Ni and Co in SNeIa is lower than that of Fe. The
decrease in [Ni/Fe] favours models of SNeIa based on the explosion of double
degenerate sub-Chandrasekhar mass white dwarfs.
