If I understand correctly, the washers don’t need constraining inwards, i.e. are free to float just can’t come off.
Would a hollow tube, loose fit on the shaft, with three punches mounted in the bottom and three guides, a sliding fit on the shaft work?
Drop a shaft in, hammer/press the top end – remove slip on two washers and put it in upside down and repeat the press action.
The guides inside the tube are needed to provide space for the indents to come out.