"Three people were killed and 11 others wounded by a roadside bomb explosion targeting their bus," a senior army officer told AFP.
"All of them were technicians from the roads and bridges department in Samarra," he said.
They were repairing a bridge just south of Samarra, whose destruction cut a vital supply line for the army, crippling operations farther north.
"Among the killed was the engineer supervising the repairs," a medical source said.
Samarra, 110 kilometres north of Baghdad, is a mainly Sunni city but also home to the Askari shrine, one of the holiest sites in Shiite Islam.