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The triplet-like QED processes $\gamma + \ell_i \to \ell^+_j \ell^-_j +\ell_i$ with $i\neq j,$ and $i=e, \mu,$ $j=e, \mu,\tau$ has been investigatedas the reactions where a dark photon, $A'$, is produced as a virtual state withsubsequent decay into a $\ell^+_j \ell^-_j-$-pair. This effect arises due tothe so-called kinetic mixing and is characterized by the small parameter$\epsilon$ describing the coupling strength relative to the electric charge$e$. The main advantage of searching $A'$ in these processes is that thebackground to the $A'$ signal is pure QED. Concerning $A'$, we consider itscontribution in the Compton-type diagrams only since, in this case, the virtualdark photon has time-like nature and its propagator has the Breit-Wigner form.So, near the resonance, $A'$ can manifest itself. The contribution of $A'$ inthe Borsellino diagrams is negligible since, in this case, the virtual darkphoton is space-like, the $A'$ propagator does not peak and the effect isproportional, at least, to $\epsilon^2$. We calculate the distributions overthe invariant mass of the produced $\ell^+_j \ell^-_j-$ pair and search for thekinematical region where the Compton-type diagrams contribution is notsuppressed with respect to the Borsellino ones. The value of the parameter$\epsilon$ is estimated as a function of the dark photon mass for a givennumber of events.

Dark photon manifestation in the triplet-like QED processes $\gamma + \ell_i \to \ell^+_j \ell^-_j + \ell_i$, $i\neq j,$ $i=e, \mu,$ $j=e, \mu,\tau$