By that time, they are basically unhatchable, sorry. That person, to put it kindly, is blowing smoke. If I were you, I'd never buy from that person again. Three week old eggs are mostly just for eating, not hatching. I have no idea who it is and it really doesn't matter. The fact is that they told you incorrect information. Most breeders try to send fresh eggs, the fresher the better. Over a week old and the hatch rate drops. Over two weeks old, it plummets. Over three weeks, it'd be almost a miracle for one to hatch.