Generally with fry-you want them to be big enough so that they can't fit in other fish mouth before transfer into the main tank-its also good to have plenty of hiding place for them if they are less than half the size of the biggest fish in the tank-just to be safe....at 3 weeks they should be big enough, however, it also depends on who is in the tank you want to transfer them to. Fish like to swallow their food whole but it may not stop them from harassing and/or killing them
In that case I think they will be fine as long as they are too big to fit in the adults mouth and healthy to start, however, if they are still really tiny you may want to give them more time to grow more.....good luck and let us know how things go