Police were called to Wealdstone High Street at 7.01pm on Tuesday, 30 November following reports of a stabbing.
Officers attended with London Ambulance Service paramedics and found two males, aged 14 and 16, with stab wounds. They have been taken to a west London hospital and their injuries have been assessed as non-life-threatening.
A third victim, a 21-year-old man, was found outside Harrow and Wealdstone Station a short time later.
He had also been stabbed and has been taken to a west London hospital where he remains in a life-threatening condition.
A significant crime scene is in place around Wealdstone High Street and an investigation is underway.
The public can expect to see a significant police presence in the area in the coming hours.
A Section 60 order, which provides officers with additional search powers to prevent further violence, has been authorised until 8am on Wednesday.
Anyone who witnessed the incident is asked to call 101 with the reference 6046/30NOV.
Information can also be provided to Crimestoppers, anonymously, on 0800 555 111.