BART police said the shooting happened at the platform where riders transfer to Antioch-bound trains. All three victims went to the hospital, though their conditions aren't known yet.
The shooting knocked out train service between North Concord and Antioch, one of BART's busier transfer points. Transit officials set up bus bridges so riders could get around the gap while the investigation continues, adding time and confusion to Monday commutes.
Investigators haven't released details on what led to the shooting or the identity of the person in custody.
This is a developing story, and BART police haven't said how long the service disruptions will last.