Ten rockets hit the southern district of Al-Hamdaniyeh, wounding eight other civilians, according to the Syrian Observatory for Human Rights.
Syrian state news agency SANA put the toll at three civilians killed and at least six others wounded.
The fire was the first rebel attack since Syria's armed forces announced Aleppo was back under their control following a fierce month-long offensive.
Under a landmark evacuation agreement, about 35,000 people were bussed out of the small pocket of residential districts in Aleppo's east, which had been held by the opposition since mid-2012.
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For more than four years, Aleppo had been divided between rebels in the east and forces loyal to President Bashar al-Assad in the west.
Government troops battered the east with air strikes and artillery fire, while opposition fighters hit the west with rockets and mortars.
Rebels have described the loss of Aleppo as a major blow for the revolt against President Bashar al-Assad.