In addition to having different package formats, CentOS, Fedora, and Ubuntu do a number of things differently as regards how to manage services and configuration routines and the like. CentOS uses System V for startup; Ubuntu uses its own thing called Upstart; Fedora uses System D.
They are all three Linux, but how they arrange information in the /etc directory, the details of where startup and configuration information is stored, and the command line tools they provide for inputting and altering configuration information differ.
I suggest you pick one distro, use it until you are comfortable with a basic knowledge of Linux, then branch out to others. It's probably a good idea to have a GUI available, even if you intend to learn how to do things from the command line; you could use the browser to look for information if you get stuck on something, for example.
You find a lot of useful references at The Linux Documentation Project