Abstract: The invention provides for the replacement of the cutting plate (15) and the press die (16) to be carried out automatically, specifically with the aid of a positioning device for positioning the stack of printed sheets (11) to be trimmed under the press die (16). In this way, the automatic conversion of the automatic three-knife trimmer for a format change is possible without a significant change to the automatic three-knife trimmer. Only the control system of the positioning device has to be changed and expanded. The positioning device (21) automatically ensures that the new press die (16) and the new cutting plate (15) are positioned exactly. In particular, the positioning of the die plate (19) under the format plate (18) can be carried out automatically by means of the positioning device with the required high accuracy, to be specific with higher accuracy than would be possible manually.