  A high statistics data sample of the decays of $K^+$ mesons to three charged
particles was accumulated by the OKA experiment in 2012 and 2013. This allowed
to select a clean sample of about 450 events with
$K^{+}\to\pi^{+}\pi^{-}\pi^{+}\gamma$ decays with the energy of the photon in
the kaon rest frame greater than 30 MeV. The measured branching fraction of the
$K^{+}\to\pi^{+}\pi^{-}\pi^{+}\gamma$, with $E_{\gamma}^{*}$ > 30 MeV is $(0.71
\pm 0.05) \times 10^{-5}$. The measured energy spectrum of the decay photon is
compared with the prediction of the chiral perturbation theory to O$(p^{4})$. A
search for an up-down asymmetry of the photon with respect to the hadronic
system decay plane is also performed.
