Results of a search for new phenomena in events with at least three photons are reported. Data from proton–proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$ , were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay $Z\rightarrow 3\gamma $ , as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be $\sigma \times {\text{ BR }}(h \rightarrow aa) \times {\text{ BR }}(a \rightarrow \gamma \gamma )^{2} < 10^{-3} \sigma _{\text {SM}}$ for 10 GeV $< m_{a} <$ 62 GeV. Limits are also presented for Higgs boson-like scalars (H) for $m_{H} > $ 125 GeV, and for a $Z'$ decaying to three photons via $Z' \rightarrow a+\gamma \rightarrow 3\gamma $ . Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR $(Z \rightarrow 3\gamma ) < 2.2 \times 10^{-6}$ , a result five times stronger than the previous result from