still kinda learning how to cut clones but pre flower stems aint long enough yet thats how young it is but will post pics later in about 8 hours time.
First, are you in the flowering phase or are you saying that your plants are 3 weeks old from seed? If you're only 3 weeks old from seed, it'll take a little time to tell their sex and like someone else mentioned, trying to confirm sex from pre-flowers is a hit or miss. It's best just to Flower a rooted clone for a week or two.
Second, if you're actually 3 weeks into flower and you still can't tell their sex, then you've probably got a major light leak coming in or they're being stressed for whatever reason only you can tell us.
Third, if they've been Flowering for 3 weeks, it's going to take much longer to root those clones simply because their hormones have changed and are concentrating on producing flowers and not roots.
Good luck with the process and keep us posted