So confused on sexing transplants?

Hey there everyone. I currently have 28 seedlings that are 1 week old in red 12 oz party cups, I am going to only use 12, and put them in 3 gallon pots. The thing I just don't get is, do I let them grow in these cups until they show they're sex, so I could weed out the males, or do I have to transplant all of them to 3 gallon potz, to tell sex?? I really need help before itz too late. thanks everyone.

let them grow in the cups untill they about 8-10 inches then transplant you wont be able to tell the sex for roughly around 40 days or soo.......

yea finding out the sex takes awhile