This is one of my females. Near the main stem you will see wispy white hairs on females, these will become bud sites. Males will form a spade shaped sac. Three weeks is probably a bit soon to tell.
This is my first grow. I have 4 plants in pre flower. I believe this is male but am not 100% sure. Can anyone with a bit more experience determine that from this picture or should I let it go a while longer?