  We present X-ray spectral and timing behavior of Cyg X-3 as observed by
AstroSat during the onset of a giant radio flare on 01-02 April 2017. Within a
time-scale of few hours, the source shows a transition from the hypersoft state
(HPS) to a more luminous state (we termed as the very high state) which
coincides with the time of the steep rise in radio flux density by an order of
magnitude. Modeling the SXT and LAXPC spectra jointly in 0.5-70.0 keV, we found
that the first few hours of the observation is dominated by the HPS with no
significant counts above 17 keV. Later, an additional flat powerlaw component
suddenly appeared in the spectra which extends to very high energies with the
powerlaw photon index of 1.49 +/- 0.04. Such a flat powerlaw component has
never been reported from Cyg X-3. Interestingly the fitted powerlaw model in
25-70 keV, when extrapolated to the radio frequency, predicts the radio flux
density consistent with the trend measured from RATAN-600 telescope at 11.2
GHz. This provides a direct evidence of the synchrotron origin of flat X-ray
powerlaw component and the most extensive monitoring of the broadband X-ray
behavior at the moment of decoupling the giant radio jet base from the compact
object in Cyg X-3. Using SXT and LAXPC observations, we determine the giant
flare ejection time as MJD 57845.34 +/- 0.08 when 11.2 GHz radio flux density
increases from ~100 to ~478 mJy.
