Add a third or fourth spool of filament to your MK3, or use a Prusa-Mini style spool holder.
This is a simple low friction method to feed filament to the extruder. 
This print works really well and is easy to install and remove. 
Print time is 1.25 hours. 
New (9-8-21): Guidering30 has a feed-in loop and uses supports for printing. 
It requires some cleanup, not at all difficult.
This is a nice improvement to the snail design, the filament cannot possibly escape.
Files include PET and PLA.
There is a small hole in the body to store the filament end when not in use. 

Guidering23 has a closed ring. Filament must be threaded through the ring from above. 
Guidering24 has an open ring. You can slip filament in or out of the ring without removing it from the extruder.

Our 2019 MK3S (factory built) has an ever so slightly thinner main frame plate compared to our 2020 MK3S (kit).  A single layer of paper draped over the frame provided the desired “grip” fit for the Guidering 23, 24 or 30 on the 2019 machine. 

The Z-Axis can move up to Z+193mm before it touches the guide. 

Also pictured, an easy to build heavy duty spool holder, held in place using arms that attach to the Mark3 frame.