My Ender-3 is running an SKR1.3 Board. This can drive two extruders. Because there is another one available wich can drive up to three extruders I already designed a completely new Joiner which can join all three filament streams together.
This Joiner can be printed as one piece. The couplings are PC4-M10. You should print with the three inlets on the bottom. To have a good transition into the single PTFE tube which goes to the hotend you have to strecht it to 3mm and then it can be put on the inner ring. This will make sure there is no edge where filament can be caught.
So far I tested it with two stream. So far it works great. Filament is feed into the joiner easily and it also pulls out good.