The system will be used to train Australian troops who are deploying to Afghanistan as part of the International Security Assistance Force.
The two systems delivered earlier by Saab are in use in Afghanistan, said Australian Minister for Defense Materiel Jason Clare.
"The Giraffe radars provide our troops with early detection of attacks from enemy rockets, artillery and mortars, protecting Australian and ISAF forces," Clare said. "This early warning system has proven to be an important force protection capability for our troops, giving them vital seconds of advanced warning so they can take shelter.
"The addition of a third Giraffe radar to train our soldiers before they deploy means they will be trained to use the full range of the radar's capabilities from the time they hit the ground."
Australia had leased Giraffe radars from Saab through a $90 million contract that includes support services.