Janet wrote:
The problem in Tina's case, I guess, is she doesn't want it to infect
her others if there is anything that can be passed on.

Well, if they came from the same source they were probably all exposed to
it before she got them

Possibly, but if they are living in a big area/shed/etc the 3 in question
may not have had much direct contact, or maybe their immune systems were
going well earlier on and the move disturbed them, or maybe the ill one
had just picked it up off a wild bird, or ... I don't know, just pointing
out that it's probably not worth the risk of putting a sick bird in with
2 healthy ones whe you don't have to.