We study the radiative B → Kη ( ) γ decays, which are important to investigate CP violation, and are also relevant to assess the role of the exclusive modes induced by the b → sγ transition to saturate the inclusive B → Xsγ decay rate. Moreover, these channels do not display the same hierarchy as B → Kη ( ) modes, for which the decay into η is enhanced with respect to one into η. The threebody radiative decays reverse the role: we find that this experimentally observed behavior (although affected by a large uncertainty in the case of the η ) is reproduced in the theoretical analysis. We compute a B * → K form factor, needed for this study, using light cone QCD sum rules, and discuss a relation expected to hold in the large energy limit for the light meson. Finally, we examine B → Kηγ in two extensions of the standard model with universal extra dimensions, to investigate the sensitivity of this rare mode to such a kind of new physics effects.