As the MMU2S to MMU3 upgrade is currently not available in the Prusa Online store, I just decied to upgrade it on my own. However, I had no 5mm flange, which is required for mounting the idler to the motor, but I had some spare 16 teeth pulleys. Thus, I just modified the original version of the idler. These pulleys are even cheaper than the flanges.
You just have to remove the one side of the pulley as shown in the images.
It is noted, that I haven't found the original step-files for the MMU3 parts, I even asked in the forum, but I got no answer: https://help.prusa3d.com/article/printable-parts-for-original-prusa_1824
The idea to use a pulley in that way is from the Voron project, especially from the enstop for the Voron 2.4: https://github.com/VoronDesign/Voron-2/blob/Voron2.4/STLs/Z_Endstop/nozzle_probe.stl
The author remixed this model.
uses 20 teeth pulley instead of 5mm flange