The structure of the Troika: Proton, Photon and Pomeron, as seen at HERA

HERA, the electron-proton collider, enables to probe the proton with a high resolving power due to the deep inelastic scattering reactions at high $Q^2$ values. In the low $Q^2$ region, one can study the properties of the photon. The large fraction of diffractive events found both in the low and high $Q^2$ region allows the study of the Pomeron. A review of what we have learned from HERA so far about the structure of these three objects is presented.