Africa's biggest steel producer said a portion of a stack at one of the operation’s coke batteries early on Wednesday fell onto a control room where the three employees were working.
ArcelorMittal South Africa, majority owned by Luxembourg-based ArcelorMittal SA, said the cause of the incident was not yet known but search and rescue teams were on site.
"A full investigation will follow to understand what happened and avoid anything like this from happening in the future,” ArcelorMittal South Africa CEO Kobus Verster said.
The National Union of Metalworkers (NUMSA) expressed concern for the workers trapped under the rubble.
The company said the coke battery was damaged in the incident and will likely be out for repairs once the extent of the damage has been determined.
Vanderbijlpark Works, located in the country's economic hub of Gauteng, is one of the world's largest inland steel mills.