Thank you, I will offer them those vegetables then, hoping that they will eat them this time.
Anyway, I let Sid, Stella and Celeste out of their cages in the same room.
However, while Sid almost immediately got out of the cage, flew in the room and interacted with Celeste without being aggressive, Stella didn't.
She stayed in the cage and even bit Celeste's toes through the bars, so I had to interrupt her out-of-cage time even before she could, well, get out of the cage.
And Celeste's room is the only room that is budgie safe.
Allowing Sid and Stella to spend some time out of their cage is harder than I thought.
Changing Stella's diet is crucial to help her lose weight, at this point.