I'll bet you transplanted directly into that 3-gallon pot, didn't you? Come on, admit it, you were too lazy to let
the plant get comfortable in a small 4-inch pot or so and went straight into the big pot.That is why growth appears so slow.
New seedlings only grow roots when first in dirt. Until their roots hit the side of the pot you will see little growth above ground. That's why we TP first into
a small pot and let it grow there a bit. Forces the plant to grow more sideways.
BigSteve.