Speaking at a news conference, the NATO chief presented his condolences to the families of the three and said an inquiry had been opened "to determine the circumstances."
The soldiers, serving with an Afghan police advisory team, were shot by a gunman wearing an Afghan National Civil Order Police uniform, the International Security Assistance Force said.
"At this stage we can't confirm that it was actually an Afghan policeman who turned his gun against ISAF soldiers, we can confirm that it was a man wearing an Afghan national police uniform," Rasmussen said.
"The Taliban has clearly laid out a strategy to undermine confidence in the Afghan national security forces, but let me also stress that they can't derail our strategy.
"Our strategy is to gradually hand over full responsibility to the Afghans and that process will continue and be concluded by the end of 2014."
The deaths on Sunday took to at least 26 the toll so far this year from 18 attacks in which Afghan forces turned their weapons against their Western allies.
Rasmussen also said he hoped that talks with Russia to transit non-lethal supplies through its territory from Afghanistan "can be concluded very soon."
"It's in their and our interest to expand the transit arrangement," he said.
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Nato you are the one who are saying that there are Good Taliban and Bad Taliban and we should negotiate with Good Taliban. Your own mantra is haunting you. So In my view there are only Taliban. No good or bad. All are same. Root out all of them so that Afghanistan and Pakistan could be a better place to live in. The main creators who funded ISI to make Taliban were America and Saudia Arabia And Saudia had already left Pakistan and is with India now a days and after creating a mess America wants to do the same so the only solution is that Nato will crush Taliban on their Afghanistan|s side and Pakistan is stepping up efforts until recently to root out Taliban from their regions and jointly you have to end this menace for good and stop fooling the people with Good and Bad types..