The deaths take the toll this year in "green-on-blue" incidents -- in which Afghan forces turn their weapons against their Western allies -- to at least 26, in a total of 18 such incidents.
In keeping with its usual policy, NATO's International Security Assistance Force (ISAF) gave no further details of the incident and did not reveal the nationality of the victims.
"An individual wearing an Afghan National Civil Order Police uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan today, killing three service members," ISAF said in a statement.
NATO has around 130,000 soldiers fighting alongside some 350,000 Afghan security personnel against the Taliban-led insurgency, but they are due to pull out of the country in 2014.
The Western coalition is to hand over security to local forces by mid-2013 and will play a support role up to the final withdrawal by the end of the following year.
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Pakistan has been trying to convince everyone who wants to know the truth that safety and security of Afghanistan is best left in Pakistan's hands. Hopefully this incidence will convince everyone that Pakistan was right.
The sooner they quit the wiser they will be
This is the same rouge army and police that the US is arming with sophisticated weapons. A great threat to Pakistan as the idea of a disciplined Afghan force is part fantasy and part wishful thinking.