Police say the stabbing happened at roughly 9:45 p.m. at 4th Avenue and Dilling Way.
It's unclear what sparked the violence, but police say a man stabbed three people before a good Samaritan tackled him and wrestled away the knife.
The witnesses kept the attacker pinned down while others called police.
One of the victims was stabbed in the stomach and is listed in serious condition at Harborview Medical Center. The second victim, also an adult man, was taken to Harborview in serious condition. The third victim was not seriously injured, but was taken to Harboriview for treatment.
The suspected attacker was also injured, but was treated at the scene before being taken to King County Jail, according to police.
Police say the man is cooperating, but they don't yet have a motive in the stabbing.