Officials say two of the victims were slashed on an "L" train pulling into Union Square around 10:30 p.m.
A 28-year-old woman was cut on her forehead and hand, and the 29-year-old man who rushed to help her was slashed twice.
The suspect ran and the victims were taken to a local hospital. The suspect, who appeared to be homeless, was described as 6'5" tall, wearing all black, official said.
About 30 minutes earlier, in a separate incident, two men got into a fight with a knife on a train arriving at Herald Square.
Officials say one of the men was stabbed in his abdomen and when the fight spilled off of the subway train, the victim was slashed again.
At this point in law enforcement's investigation, the attacks are not related. No arrests have been made.
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