Cosby - no, way too neat. Cher maybe. Burt looks a little too neat also. I am going by my IPs for all 3 signed on the street, so I dont know much about other circumstances. Id say the worst one of them is the Cosby
Bill Cosby is secretarial which is strange because secretary usually just signed pictures his office provided while he, Bill, would authentically sign the items fans sent in which I would imagine this item would have been. The main difference between a real versus secretarial Cosby is she forms all the letters while he forms most of the letter until portion of last name because a few up and down lines.
The Burt is iffy. It appears to be his writing but too neat, as mentioned, compared to most of his autographs. I have noticed he could sign pretty nicely at times so maybe he wanted to give that unique photo a neat signature.