Gbd says the amendment can work in 3 gallon containers but he reccomends and prefers 5. I’d guess that the times it worked in a 3 the plants grown were not larger varieties like the hubba bubba.
I’m trying it the first time myself right now but in a 5. No complaints so far, but I’m only 4 weeks in at the moment. I mixed 1/3 of a cup into the bottom half of the container along with a heaping handful of worm castings. Also tossed a handful of castings into the top half of the container with the unamended roots organic soil.