It appears that the license terms themselves are essentially the same, with the exception of a longer explanation for clause 1 in the zlib license and a lengthier disclaimer statement in the libpng license. Also, the libpng license includes the applicable attribution notices for the various project versions.
Fedora treats these two licenses as functionally identical, and calls them both "zlib".
The third license looks to just be a templated version of the zlib license.
