The leak is very minor and does not pose any danger, Vera Medvedkova, spokeswoman for Russian Mission Control, told The Associated Press.
"This leak doesn't present any kind of danger for the landing of this crew, and the landing will be carried out according to plan," Vladimir Solovyov, the chief of Russia's Mission Control, was quoted by the Interfax and ITAR-Tass news agencies as saying.
The helium is used to pump fuel into the craft's engine. Solovyov told Interfax that the leak was found several months ago, but that specialists didn't consider it significant enough to delay the landing.
"Similar small leaks of helium have been found earlier in other Soyuz crafts, but they have had no effect on the cosmonauts' return to Earth," Solovyov said. "In all similar cases, the landing has been successful."
Rosaviakosmos, Russia's space agency, said that the leak was so insignificant that it shouldn't have any effect on the craft.
The Soyuz TMA-3, which has been in space for six months, is scheduled to return two International Space Station residents, American astronaut Michael Foale and Russian cosmonaut Alexander Kaleri, to Earth on Friday. European Space Agency astronaut Andre Kuipers of the Netherlands will also be returning after a nine-day mission on the station.
The U.S. manned space program has been entirely dependent on Russia's Soyuz crafts since the grounding of U.S. shuttles following the Columbia disaster in February 2003.