  Azimuthally-differential femtoscopic measurements, being sensitive to
spatio-temporal characteristics of the source as well as to the collective
velocity fields at freeze out, provide very important information on the nature
and dynamics of the system evolution. While the HBT radii oscillations relative
to the second harmonic event plane measured recently reflect mostly the spatial
geometry of the source, model studies have shown that the HBT radii
oscillations relative to the third harmonic event plane are predominantly
defined by the velocity fields. In this Letter, we present the first results on
azimuthally-differential pion femtoscopy relative to the third harmonic event
plane as a function of the pion pair transverse momentum $k_{\rm T}$ for
different collision centralities in Pb-Pb collisions at $\sqrt{s_{\rm
NN}}=2.76$ TeV. We find that the $R_{\rm side}$ and $R_{\rm out}$ radii, which
characterize the pion source size in the directions perpendicular and parallel
to the pion transverse momentum, oscillate in phase relative to the third
harmonic event plane, similar to the results from 3+1D hydrodynamical
calculations. The observed radii oscillations unambiguously signal a collective
expansion and anisotropy in the velocity fields. A comparison of the measured
radii oscillations with the Blast-Wave model calculations indicate that the
initial state triangularity is washed-out at freeze out.
