Why do you keep plucking out some of the seedlings? Are they all feminized? It isn't about picking your strongest 'unsexed' seedlings, it's about maximising your chances of yielding some bud at the end of a 3-4 month process.
You started with a dozen seedlings, you now have three. If they're all bag seed, how many of the original dozen did you think would turn out female? All of them?! If you've somehow managed to inadvertently pull out all the male seedlings, and the three you're left with turn out to be female, you should probably go out and buy a lottery ticket, you know, because you're so lucky and stuff.