Right. They say that rsh can be used to get a login or run a command on a remote host. So why do the other two commands exist? In what situations would a person have wanted to use rlogin or rexec instead of rsh?
Maybe rexec appeared first? And when rsh arrived later, they still kept rexec around? Perhaps they had a lot of shell scripts that contained "rexec" and didn't want to bother changing them to "rsh"? The actual reason is probably so mundane that maybe there is no accurate historical information on this topic.