Changing the filaments on an MMU3 can be a real pain, especially if try to do it with the Idler Body closed. When the filament is fed into the MMU3 Pulley Body, it may not align with the pulleys, and the Idler Body bearings may not redirect it onto the proper path. Wrangling the filament onto the right path is a fight, as access is limited, and the Selector may be in the way. There is a much simpler way.  

Opening the Idler Body provides good access to direct filaments along their desired paths. Using a 2.5mm Allen Key for the Idler Tension screws, however, is inconvenient and uncomfortable. Prusa should have designed this mechanism using captured thumbwheels.

This model, when used with a spare 2.5 mm Allen Key, creates a tool that makes opening and closing the Idler Body quick and easy 

Cut down a spare 2.5 mm Allen key to approximately 3cm, and glue it into the thumbwheel. Create a pair to open and close the Idler Body with little problem. 

Keep these at the ready for when you need to make a filament change. If you have the Prusa Enclosure, affix them to the lid with magnets.