All three shows, which still have huge followings, were created by Joss Whedon and were lauded during their runs for the quality of their writing.
The rally at Twentieth Century Fox Studios was organized through a number of Internet fan sites dedicated to Whedon's shows, said a statement posted on the Writers Guild of America Web site.
"For the first time, we're able to make the studios aware the fans are in the equation," Whedon said. "They're the third part of the negotiation. They get behind worthy causes and storytelling and the basic rights of workers are worthy."
"We have to support the writers so we can have real things on TV. Things worth watching," said Lara Field, one of a group of eight fans who flew from Australia for the rally.
Whedon players Eliza Dushku, Nathan Fillion, Summer Glau, Ron Glass, Nicholas Brendan, Tom Lenk, Juliet Landau, J. August Richards, Camden Toy, Amy Acker and Michael Fairman all took part in the event.