your 3 weeks in, your buds will fatten up but not over night, i am sure you will see those buds fatten up it will just take a few weeks to get them there. its like anything else
with cannabis, you start from seed, and it seems like the plant is too small and we wait for the size to come, then since it is an auto we see flowers form and say man why aren't my buds getting bigger, then when we harvest we say wow i got more than i think. long story short, good things take time, the 2-3 weeks in flower not much happens, 4-6 weeks we see buds slowly get fat, 6 weeks until harvest those buds just get huge.
but autos run on their own time frame thats been breed in to them , they just need time.