They said a woman, a child and a young man were killed and at least eight others injured when one exploded near a public park in the heart of the city of al-Bab, north of Aleppo.
"There were a lot of injuries.. it was a motorcycle (that exploded) near a main roundabout..," said Ibrahim Sabra, a trader in the vicinity of the blast.
A local police source in the town said they had staged a controlled detonation of another motorbike in the town of al-Rai, north of al-Bab, and arrested a suspect.
In a similar incident, three people were injured in the nearby towns of Qabasin and al-Ghandura by blasts also caused by explosives planted in motorcycles parked in public places.
On Wednesday two explosive devices detonated in the city of Afrin, a mainly Kurdish area which Turkish forces and their Syrian allies took from the Kurdish YPG militia last year.
Rebels accused the YPG of carrying out the blasts, which they say seek to sow fear in areas where Turkey has carved a sphere of influence with the help of Syrian Arab rebels it backs and arms.
There was no immediate comment from the YPG, which has denied previous accusations of attacks on civilians, saying it only targets Turkish soldiers and their allies.
Turkey regards the YPG as a terrorist group tied to the PKK inside its own borders, and has staged incursions into Syria in support of Syrian rebels to push it from the Turkish frontier.