You hurt yourself by not transplanting into a small pot first.
When seedlings first go into dirt the plant uses most all it's energy growing a rootball to support the plant.
Until the roots hit the inside walls of the pot you will see little or no growth above ground.
That is a mistake here because autos sprout, grow, flower and die in just 2-3 months. You put the seedling
directly into a 3+-gallon pot right off the bat so she is wasting weeks building a much bigger rootball than
necessary. Better to put the newly germinated seedling into a solo cup for 10-14 days, then into another smallish 4-inch pot
for 2 more weeks. By going into slightly larger pots you aren't wasting so much time growing unneeded roots.
BigSteve.
PS -- This is an advanced situation for a new grower so this is what I would do. First, it's too late to TP into a smaller pot
so leave her where she is. Autos do not veg, you know, they go straight from seedling to flowering mode. So most
authorities say to use a 20/4 light schedule for all autos. In this case though you want to slow down the maturation process
as much as possible so your gal can grow before flowering. So turn the lights to 18/6 instead of 20/4. The longer light period should
push her to grow more than with the lights at 20/4.
PM me if you have any specific questions. Good luck, BigSteve.