All three can be used to take some text as input, process (transform) it in some ways and return it as output (which can be written to file or pipe to another program for further processing). I usually like to describe them as "text processors".

Perl is a lot more than just a text processor like the other two. It is a general purpose programming language with which you can write programs to do a wide variety of tasks. There are many third-party Perl libraries (modules) that gives you many convenient functionalities.