Combine the teff flour and 12 cups water. Mix, cover and store in a dark, cool place for 3 to 4 days to ferment, giving it its sour flavor.
On the last day of fermentation, mix the teff flour mixture with the barley and wheat flours. Let rest for another 8 hours.
Heat a large cast-iron pan over medium heat. Pour 1 cup fermented mixture into it, swirling the pan to cover the entire surface area. Cover with a lid, 2 to 3 minutes. Injera is ready to plate when holes have formed on the surface.