A mother, her daughter and another girl died when the mortar detonated in the Sorkhrod district of Nangarhar Province, the Nangarhar authority media office said.
Eight other people were injured, including two girls, it said.
The mortar is believed to have been fired by Taliban militants on Saturday, with the deadly explosion occurring Sunday at 7 a.m. local time (11 p.m. Saturday ET).
The Taliban has been waging a bitter fight in Afghanistan with the ultimate goal of ruling the country and imposing its strict interpretation of Islamic law. The group controlled Afghanistan until 2001, when it was overthrown by the US-led coalition that invaded the country following the 9/11 attacks.
In recent years, a resurgent Taliban has taken control of significant swaths of the country.
While Sunday’s explosion is thought to be the result of a mortar fired just a day earlier, decades of conflict have left a dangerous legacy in Afghanistan.
Mine-clearing organization Halo says Afghanistan has been left littered with unexploded ordnance, with more than 23,500 casualties recorded between 1979 and 2015.
Last year, the UN Assistance Mission in Afghanistan said the number of child casualties had jumped in 2016 — spiking 24% from 2015 — in large part from leftover munitions.
“Children have been killed, blinded, crippled — or inadvertently caused the death of their friends — while playing with unexploded ordnance that is negligently left behind by parties to the conflict,” said Zeid Ra’ad Al Hussein, UN High Commissioner for Human Rights in a February 2017 report.
In 2012, the Mine Action Coordination Centre of Afghanistan said unexploded ordnance accounted for three times as many casualties as mines.