A method for operating a three side trimmer comprising: providing a trimming unit trimming three sides of one or more booklets; providing a discharge unit arranged downstream of the trimming unit to discharge the trimmed booklets, an entrance of the discharge unit being arranged at a height equal to or lower than an exit of the trimming unit; providing an accumulating unit arranged between the trimming unit and the discharge unit and operative to switch between a delivery mode in which the accumulating unit delivers the trimmed booklets to the discharge unit at every accumulation of a predetermined number of the trimmed booklets and an accumulation mode in which the accumulating unit accumulates the trimmed booklets without delivering the trimmed booklets to the discharge unit, the accumulating unit having a shelf movable in a vertical direction, the trimmed booklets being placed on the shelf, an elevating mechanism moving the shelf up and down, and a conveyor belt or a drive roller conveyor arranged on the shelf and adapted to supply the trimmed booklets placed on the shelf to the entrance of the discharge unit; and providing a conveying mechanism conveying the trimmed booklets from the trimming unit to the accumulating unit each time the trimming operation is completed, wherein during operation of the accumulating unit in the delivery mode, the shelf of the accumulating unit is lowered from a height equal to the exit of the trimming unit by a distance corresponding to a thickness of the trimmed booklets at every placement of the trimmed booklets on the shelf, and when the predetermined number of the trimmed booklets are accumulated on the shelf, the shelf is moved to a height equal to the entrance of the discharge unit and stopped in a manner such that the conveyor belt or the drive roller conveyor supplies the predetermined number of the trimmed booklets to the discharge unit, thereafter the shelf is moved to the height equal to the exit of the trimming unit so as to receive the next trimmed booklets, and during operation of the accumulating unit in the accumulation mode, the shelf of the accumulating unit is lowered from the height equal to the exit of the trimming unit by a distance corresponding to the thickness of the trimmed booklets at every placement of the booklets on the shelf until the number of the trimmed booklets accumulated on the shelf reaches the maximum number of the trimmed booklets that can be accommodated by the accumulating unit or a predetermined numerical value. providing a trimming unit trimming three sides of one or more booklets; providing a discharge unit arranged downstream of the trimming unit to discharge the trimmed booklets, an entrance of the discharge unit being arranged at a height equal to or lower than an exit of the trimming unit; providing an accumulating unit arranged between the trimming unit and the discharge unit and operative to switch between a delivery mode in which the accumulating unit delivers the trimmed booklets to the discharge unit at every accumulation of a predetermined number of the trimmed booklets and an accumulation mode in which the accumulating unit accumulates the trimmed booklets without delivering the trimmed booklets to the discharge unit, the accumulating unit having a shelf movable in a vertical direction, the trimmed booklets being placed on the shelf, an elevating mechanism moving the shelf up and down, and a conveyor belt or a drive roller conveyor arranged on the shelf and adapted to supply the trimmed booklets placed on the shelf to the entrance of the discharge unit; and providing a conveying mechanism conveying the trimmed booklets from the trimming unit to the accumulating unit each time the trimming operation is completed, wherein during operation of the accumulating unit in the delivery mode, the shelf of the accumulating unit is lowered from a height equal to the exit of the trimming unit by a distance corresponding to a thickness of the trimmed booklets at every placement of the trimmed booklets on the shelf, and when the predetermined number of the trimmed booklets are accumulated on the shelf, the shelf is moved to a height equal to the entrance of the discharge unit and stopped in a manner such that the conveyor belt or the drive roller conveyor supplies the predetermined number of the trimmed booklets to the discharge unit, thereafter the shelf is moved to the height equal to the exit of the trimming unit so as to receive the next trimmed booklets, and during operation of the accumulating unit in the accumulation mode, the shelf of the accumulating unit is lowered from the height equal to the exit of the trimming unit by a distance corresponding to the thickness of the trimmed booklets at every placement of the booklets on the shelf until the number of the trimmed booklets accumulated on the shelf reaches the maximum number of the trimmed booklets that can be accommodated by the accumulating unit or a predetermined numerical value.