When NASA lost its capacity to reach space with the retirement of the 30-year shuttle program in 2011, Orbital Sciences and SpaceX stepped in to fill the void with their privately-made supply spacecraft.
Both companies have lucrative contracts with the US space agency to ferry supplies to the ISS.
SpaceX, the California-based company owned by Internet entrepreneur Elon Musk, became the first private company to reach the ISS in 2012 and has a $1.6 billion contract for 12 missions with its Dragon space capsule.
Orbital has a $1.9 billion contract with NASA for eight cargo missions to the ISS, toting supplies, food, science experiments and spare parts.
The last time astronauts stepped out to replace a faulty ammonia pump was in 2010, and the repair also took three spacewalks to complete.
The December trio of spacewalks will mark the first time astronauts have ventured outside the orbiting outpost since a helmet leak brought an early end to a European astronaut?s outing in July.
Italian Luca Parmitano was wearing a US-made spacesuit when a water leak flooded his helmet and risked drowning him.
Two Russians aboard the station are scheduled to step out on a spacewalk for maintenance work on their own module at the lab on December 27.
Commander Oleg Kotov and Flight Engineer Sergey Ryazanskiy are slated for the outing to install cameras and other equipment on a platform outside the Russian segment of the station.