Depends on the size of container/tank as well as the size of the spawn. Obviously 3 weeks old would not hurt the eggs however the waste come from the feeding of the 3 weeks old may become the problem for eggs. Since eggs does not need anything but clean water and oxygen to develope. Also the chemical you may use to avoid the fungus to the eggs is not good for the hatched fry.
So if I were you, I would get some small tapperware or something and put the eggs in it with some air tube and float or attach to the top of the parents tank. That way, you can use fungcide if you need to and no worry of fouling water from the feeding. And once they hatch and absorb the egg sack you can put them with previous frys to grow them together. That way you can feed same time.
