All three soldiers killed on Thursday in Kidal were from Guinea, the Guinean government said. The identity of the attackers was not yet known.
The UN camp came under heavy mortar fire that also wounded five other peacekeepers. Shortly afterwards, the three troops were ambushed by gunmen just outside the base. No further details were provided.
Peacekeeping forces face frequent attacks from desert fighters who have regrouped since a French-led military operation in 2013 to drive them out of Mali's northern towns.
Despite ongoing French air raids on their hideouts, fighters - including local al-Qaeda cadres - have staged a series of attacks this year, including a suicide bombing at an army base in January that killed at least 77 people.
Several French soldiers were wounded last week in an al-Qaeda mortar attack in Timbuktu.
More than 100 soldiers have died in recent months, making it the most deadly UN mission to date.
France has sent some 4,000 soldiers to the region, and 11,000 UN peacekeepers have been deployed to ensure Mali's stability.
However, UN's forces have lacked equipment and resources, making a political settlement between Tuareg rebels and the government increasingly fragile.
The country is at the centre of the troubled Sahel region and is among the world's poorest. Unemployment is rampant and most people survive hand to mouth.