They are all different in subtle ways, although they also have a lot in common. For instance, Fedora uses RPM files for package management, while Ubuntu uses .deb files. I don’t know what CentOS does. If you’re just experimenting with them, it probably doesn’t matter which one you choose. If you’re setting up some kind of corporate or network infrastructure with them, some are likely to be better or worse than others in terms of administration and maintenance.