There are no other conditions. Bring three humans within seven days.
Did you bring them all during the day? and then not talk to them?
If so, go back at night and talk to them.
*Edit. Did you know that you can only capture one at a time? So you can't go fight three human / humanoids then drop all three of them off. Because you will only be dropping off one. You can't carry three hostages on your person, only one. You should see a warning when you already have one creature in your possession asking to be sure that you want to replace the new one with the old one. (effectively releasing the one you don't keep.)
So you have to fight one, capture it, deliver it to the cemetery, fight another one, capture it, deliver it to the cemetery, fight another one, capture it, deliver it to the cemetery and then talk to the vampires in the cemetery in the night after you have delivered all three.