  We present Low-Frequency Array (LOFAR) 143.5-MHz radio observations of
flaring activity during 2019 May from the X-ray binary Cygnus X-3. Similar to
radio observations of previous outbursts from Cygnus X-3, we find that this
source was significantly variable at low frequencies, reaching a maximum flux
density of about 5.8 Jy. We compare our LOFAR light curve with contemporaneous
observations taken at 1.25 and 2.3 GHz with the RATAN-600 telescope, and at 15
GHz with the Arcminute Microkelvin Imager (AMI) Large Array. The initial
143.5-MHz flux density level, $\sim$2 Jy, is suggested to be the delayed and
possibly blended emission from at least some of the flaring activity that had
been detected at higher frequencies before our LOFAR observations had begun.
There is also evidence of a delay of more than four days between a bright flare
that initially peaked on May 6 at 2.3 and 15 GHz, and the corresponding peak
($\gtrsim$ 5.8 Jy) at 143.5 MHz. From the multi-frequency light curves, we
estimate the minimum energy and magnetic field required to produce this flare
to be roughly 10$^{44}$ erg and 40 mG, respectively, corresponding to a minimum
mean power of $\sim$10$^{38}$ erg s$^{-1}$. Additionally, we show that the
broadband radio spectrum evolved over the course of our observing campaign; in
particular, the two-point spectral index between 143.5 MHz and 1.25 GHz
transitioned from being optically thick to optically thin as the flare
simultaneously brightened at 143.5 MHz and faded at GHz frequencies.
