Police were notified at 10.15 A.M. (0815 GMT) that a man had opened fire at the school with an automatic rifle, the police spokesman for Una-sana canton, Adnan Beganovic, said.
The shooter killed the school dean, the secretary, and a teacher, he said, Reuters reported.
"He tried to commit suicide and was gravely injured," Beganovic said, adding that the suspect was transferred for emergency treatment in the nearby town of Banja Luka.
An investigation is underway, he said.
The school had not yet reopened from the summer holidays so no children were involved.
N1 TV, citing witnesses, said that a janitor who had a history of disagreements with the management and was under disciplinary proceedings sought out specific people and shot them. Reuters could not immediately verify that report.
Mass shootings are comparatively rare in the Western Balkans which is awash with weapons that remained in private hands from wars in the 1990s.
In July, a war veteran in neighbouring Croatia shot five people including his mother in a nursing home and wounded six others.
SD/
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