Commentary Purg IX 111

Dante's threefold beating of his chest has been glossed, since the time of the Ottimo (DDP Ottimo.Purg.IX.106-111), as signifying 'mea culpa, mea culpa, mea maxima culpa.' The author of that commentary proceeds, followed in this, as well, by many later commentators, by reporting that these three in turn signify three kinds of sin: sins of thought, of the tongue, and in deed.