The suspect in the shooting in suburban Bethesda was at large, police said on Twitter.
The initial call to authorities came in at 11:13 a.m. as an assault outside of the mall, Montgomery County police spokesman Rick Goodale said.
The shooting victims - two male and one female - were taken to a hospital, Goodale said.
Montgomery County Fire and Rescue said on Twitter that several injured people were transported.
Montgomery County noted on Twitter reports of another shooting at a shopping center in nearby Aspen Hill. It was not immediately clear if the two shootings were related.
Witnesses say they saw gunman walking away, 3 ppl on ground after shooting outside Mont. Mall @nbcwashington pic.twitter.com/06V8N5xJ16
- Meagan Fitzgerald (@MeaganNBCDC) May 6, 2016
(Reuters reporting By Ian Simpson and Suzannah Gonzales; editing by Daniel Trotta and Bill Trott)