  In September 2016, the microquasar Cygnus X-3 underwent a giant radio flare,
which was monitored for 6 days with the Medicina Radio Astronomical Station and
the Sardinia Radio Telescope. Long observations were performed in order to
follow the evolution of the flare on a hourly scale, covering six frequency
ranges from 1.5 GHz to 25.6 GHz. The radio emission reached a maximum of 13.2
+/- 0.7 Jy at 7.2 GHz and 10 +/- 1 Jy at 18.6 GHz. Rapid flux variations were
observed at high radio frequencies at the peak of the flare, together with
rapid evolution of the spectral index: alpha steepened from 0.3 to 0.6 within 5
hours. This is the first time that such fast variations are observed, giving
support to the evolution from optically thick to optically thin plasmons in
expansion moving outward from the core. Based on the Italian network (Noto,
Medicina and SRT) and extended to the European antennas (Torun, Yebes, Onsala),
VLBI observations were triggered at 22 GHz on five different occasions, four
times prior to the giant flare, and once during its decay phase. Flux
variations of 2-hour duration were recorded during the first session. They
correspond to a mini-flare that occurred close to the core ten days before the
onset of the giant flare. From the latest VLBI observation we infer that four
days after the flare peak the jet emission was extended over 30 mas.
