A simple extended Huckel type molecular orbital model is used to study electronic distributions in mixed-valence iron-oxygen clusters containing three octahedral subunits. Two different structures were considered : one-atom bridged octahedra and octahedra with common faces. The stability of symmetric structures relative to different non-totally symmetric distortions leading to the vibronic charge trapping was examined. It appears that the distribution of the excess electron cannot be described by the simple alternative "localization-delocalization". The type of electronic distribution depends on the arrangement of octahedral units, the metal-metal distances, and the electronic configurations. The molecular orbital approach gives some insights into the peculiarities of the intramolecular electron delocalization in mixed-valence iron-oxygen trimers which are important for their catalytic activity.