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.
NEWS: NASA Picks SpaceX to Lease Old Shuttle Launch Pad
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.
PHOTOS: SpaceX Grasshopper Rocket Takes Giant Leap
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.