The incident happened in Northolt Road at around 4pm today (March 5).
Officers and London Ambulance Service crews attended and found a male who had been stabbed.
Two further male victims were also found injured nearby.
They were all taken to hospital for treatment where their conditions are currently unknown.
A number of crime scenes are in place and officers remain in the area.
No arrests have been made as police enquiries continue.
A spokesperson for Metropolitan Police: “Police were called at about 16:00hrs on Wednesday, 5 March to reports of a male stabbed on Northolt Road in Harrow.
“Emergency services attended. Two further male victims were found nearby.
“All three have been taken to hospital by the London Ambulance Service. We await an update on their injuries.
“A number of crime scenes are in place and officers remain in the area.
“At this early stage there have been no arrests. Enquiries continue.
“Anyone with information is asked to call police via 101 quoting CAD 4971/05Mar.”
