Peculiarly enough,
Cartoon Network cannot seem to decide on the spelling of these three's communal name -- "Rowdyruff Boys" turns up 8 hits on cartoonnetwork.com, while
Rowdy Ruff Boys gives 171 (though many of them seem to be unrelated to the characters at all). Google turns up 652 for the former and 669 for the latter, including the two-word spelling on both
Barnes & Noble and
Scholastic's descriptions of
The Powerpuff Girls Ruff `n´ Tuff Tattoo Book (
ISBN 0-439-16016-2) even though the one-word spelling is used on the commercially available videotapes.
In the cartoon, Mojo Jojo means to create evil little boys to fight the Powerpuff Girls, but he is in prison at the time and doesn't have access to Chemical X, with which the girls were given their superpowers -- he has to use what he's got. Therefore the boys are actually created from armpit hair, escargot, a puppy's tail, and the contents of the prison toilet. This may explain why they try to beat Mojo up before he explains to them that the girls deserve the animosity more.
Sources: search results as described above and
http://www.bcdb.com/bcdb/detailed.cgi?film=13092