I know this is probably not the solution you want to hear, but you need to separate the third bird and either find it a new home, or keep it separated from the parents. You are very fortunate that the parents have not attacked the young bird. Generally when the parents are ready to breed again, they will start to attack any weaned offspring who are still around. Probably the only reason this has not happened is that you have a large cage and let them out.
There are several reasons why this bird cannot remain with them. Related birds should not be allowed to breed. Hopefully this bird did not mate with the mother. When related birds are bred, you will end up with weak or deformed chicks or chicks with genetic flaws. Even though the parents are currently tolerating this nesting behavior from the young bird, this is not a safe or healthy situation. It is very possible that one or both parents will finally become territorial and attack the young bird. They can seriously injure or kill the bird. If any chicks hatch, the parents are even more likely to demonstrate this behavior. This can be a confusing situation for them and the eggs could get destroyed, any one of the birds might decide to eat the eggs, or if chicks hatch, the chicks might be attacked and killed. Please remove this third bird before something terrible happens. It can come with no warning. Typically when there is an odd bird, it will even raid a nest and destroy everything. I know things seem fine now, but at some point this year old bird will start demonstrating true cockatiel hormonal behavior and fighting is going to start.
He needs his own cage, and should not be close to the other birds while they are breeding and raising chicks. If the breeding cockatiels decide he is a threat, they may fight with each other when they can’t get to him, or even turn on their own chicks. I know none of this seems likely with your current situation but please let me stress that this is not normal behavior and is not going to last. If you plan to keep breeding your birds, you need to be able to find homes for the chicks when they are weaned. You can’t keep related birds together if they are opposite sexes. Brothers and sisters will breed as will parents and mature offspring. So please take all of this into consideration. Most pet stores will buy young cockatiels or give you some store credit. If you still want to keep them, again, do not keep the related birds in the same cages unless they are the same sex.
Thank you for asking Lafeber,