The problem is having a third bird in the cage. If you want to breed birds, you can only have one pair in the cage. Having two females was too distracting for the male, and probably much too stressful for the female with eggs. In the wild, another adult cockatiel would never be allowed anywhere close to the nest. Another cockatiel is a rival and would be chased away. Their instinct is to breed and raise their own chicks, so extra birds in a cage with breeders can result in them raiding the nest and destroying eggs or killing the chicks. They can also start to fight viciously and generally will kill any extra birds in the cage. So you are very lucky the extra female wasn’t attacked and killed. Right now, you need to separate the pair – it is not a good idea to have an odd number of birds in a cage anyway. You can get the other female a mate, but they still must be in a separate cage. For the pair, wait 6 months before giving them a nest box again. They need this time to rest so the female can rebuild her health after laying eggs. You need to remove the box and rest them for 6 months after every clutch. In 6 months, give the nest box back to them and maybe they will do better without the extra bird in the cage. The parents are supposed to take turns sitting on the eggs – one sits during the day and one sits at night. The male may have felt like he needed to guard against the extra female.
Thank you for asking Lafeber,