REXX is an interpreted programming language first seen on IBM systems. Python is an interpreted programming language by Guido van Rossum*. TCL is John Ousterhout*'s embeddable command language, designed just for embedded command extensions, but lately used for larger applications. TCL's most intriguing feature for many people is the tcl/tk toolset that allows for interpreted X-based tools. Others use it for its ``expect'' extension.
To avoid any flamage, if you really want to know the answer to this question, probably the best thing to do is try to write equivalent code to do a set of tasks. All three have their own newsgroups in which you can learn about (but hopefully not argue about) these languages.
To find out more about these or other languages, you might also check out David Muir Sharnoff*'s posting ``Catalog of Compilers, Interpreters, and Other Language Tools'' which he posts to comp.lang.misc, comp.sources.d, comp.archives.admin, and news.answers newsgroups. It's a comprehensive treatment of many different languages. (Caveat lector: he considers Perl's syntax ``unappealing''.)
Other resources at this site: