I used to breed U2's and G2's and have always had a soft spot for cockatoos. However, I currently don't have any as pets. They are simply too demanding for me at this phase of my life. Don't get me wrong, they can be wonderful pets for the right person in the right situation. It is good to research first though.
As for a 3 month old being weaned - what type of 'too is it? My babies were never weaned at 3 months. The majority of goffins weaned about 4 months sometimes 5; umbrellas 5 to 6. Of course, I didn't foce wean, and still don't so each bird was treated as an individual and allowed to wean at their own pace.
Please also know that baby cockatoos will sometimes revert to "begging" behavior when they are moved. This has to do with insecurity more than anything although sometimes you will get a baby that isn't completely weaned, or has not been weaned properly. In this case the baby is hungry and you should be able/willing to offer either formula from a spoon, or soft warm foods from your fingers to ease the transition which can take quite awhile. Also, cockatoos do not eat as much as an amazon or pionus and can literally starve themselves if their needs are not met (i.e. the new owner gets a baby and doesn't realize the baby is hungry).
They are very sensitive creatures and in my opinion the vast majority of people really are not qualitifed to own them. This is why I stopped raising them. Too many end up in rescues.
Sorry for the novel.
Sally
Idaho