Consider the set of 1PI diagrams with three external photons.
It is apparent that there are no tree-level diagrams for this process.
What are the possible one-loop diagrams for this process? Why do the different one-loop diagrams involve simply an interchange of the Lorentz indices and photon momenta?
Should this diagram be interpreted as a $1 \to 2$ scattering or a $2 \to 1$ scattering?
What about $4$-photon scattering? Should a $4$-photon scattering be thought of as a $1 \to 3$, $2 \to 2$ or $3 \to 1$ scattering?