It’s not a good idea to have an odd number of birds in one cage. And when you have several birds in a cage, they should never be given nests or nest boxes. I would remove the lonely female and her nest and eggs right away. She is in danger of being killed by the pair that is bonded. Lovebirds get very territorial and aggressive when they are breeding. You will not be able to return this female to the cage. You can find her a home or get her a male, but whatever you decide, she needs to be taken out of this group cage now. You can’t have more than one pair in a cage, either. They will end up fighting and you will most likely end up with dead birds. Also, they may raid each other’s nests and destroy the eggs or kill any chicks that hatch. Birds do not need a nest or nest box unless you want them to breed. With the pair, whether the eggs hatch or not, remove the nest once they are done with the eggs or once any chicks leave the nest. Do not give them the nest again for 6 months so they can rest. For the single female, throw her eggs away and do not give her a nest. Single birds should not be given a nest because it causes them to lay eggs. It’s not healthy for a hen to lay eggs too often. So this is why you have to remove the nest and make them rest.
Thank you for asking Lafeber,