Hi Adiv,
Removing the nest box will help with the dynamics. And since they are loose all day, this makes it less likely that the odd bird out would get attacked. It is more likely to happen if they are all locked in a cage together, and he can’t get away from the other male or female. You might get a small cage as a sleep cage for the single bird – this would remove any risk of him getting attacked.
You can make some changes to discourage more egg laying. Keep in mind that to lay eggs, she needs longer daylight, warmer weather, abundant food, and a quiet, private environment. Your goal is to reverse these conditions.
Limit her light to 8-10 hours by covering the cage early each evening
Do not give her anything to use as a nest – no bird huts or tents, no box, bowl, etc. If she decides to sit in a food bowl, remove it and replace with smaller cups.
Do not give her anything to shred such as paper or cardboard.
Rearrange the toys in the cage frequently.
Usually we say move the cage to a different place in the room, but I know they are out most of the day. You can still try it, though. Move the cage about once a week, or whenever she shows signs of nesting – settling on the cage floor for example.
If she is let out of the cage, do not let her get in any dark cozy places and don’t give her free roam. Of course she has free roam all day, but at least try to keep her out of places she might want to nest.
If she is tame and you handle her, limit any petting to her head and neck – do not pet her on the body.
Thank you for asking Lafeber.
Brenda