Officials said the aircraft would be operated by the Qatari air force to perform emergency medical service missions.
The deal follows a similar contract signed in 2008 for 18 AW139 aircraft, including crew training and a spare package. With the deal the Qatar armed forces became the third operator of military-configured AW-139 helicopters, following the Irish air corps and the United Arab Emirates armed forces.
Several helicopters have been delivered and used by various government agencies to perform a range of roles including utility, troop transport search-and-rescue, border patrol and Special Forces operations. The rest are due to be delivered by early next year.
Designed to be easily and quickly converted between roles, AgustaWestland said it prides the AW139 as being "perfectly suited for military and public security applications."
It says the helicopter's cabin can be configured to carry up to 15 troops while the size of its sliding cabin doors allow both troops and equipment to be loaded and unloaded easily and quickly.
Giacomo Saponaro, senior vice president for the International Government Business Unit for AgustaWestland said in a company statement that the contract provided "clear evidence of customer's satisfaction with the best selling multi-role medium twin helicopter."
The order, he added "further expands the incredible success achieved by the AW139 in Qatar for a variety of applications and makes this market one of the most important for the type in the Middle East with almost 40 units so far ordered."
At least 440 orders have been placed by more than 120 customers from some 50 countries, making it "the benchmark helicopter "in its category, AgustaWestland said in a statement.
With a maximum cruise speed of 165 knots and a maximum range beyond 570 nautical miles, the AW139 is believed to have set new standards of performance in its class for military applications.
There was no immediate confirmation on the cost of the three new helicopters but the initial deal for the purchase of 18 AW139s reached $300 million.
Qatar was the first Arab nation to officially announce its participation in the enforcement of a United Nations-backed no-fly zone over Libya.