KABUL—A roadside bomb hit a bus carrying university staff in northern Afghanistan on Saturday, killing three teachers and wounding 15 others, police said on Saturday. The incident took place in Charikar, the provincial capital of Parwan. The bus was carrying teachers from Al-Biruni university, said a spokesman for the provincial police, Salim Noori. Some of the wounded teachers were in critical condition, said Hamed Obaidi, a spokesman for the ministry of higher education. No group claimed responsibility for the incident. Roadside bombs, small magnetic bombs attached under vehicles and other attacks have targeted members of security forces, judges, government officials, civil society activists, and journalists in recent months in Afghanistan. The government usually blames the Taliban for such attacks but the insurgent group denies involvement. Violence has sharply increased since Washington announced plans last month to pull out all U.S. troops out of Afghanistan by Sept. 11. Three weeks ago, …