Emergency services were called to Sourton Down, near Okehampton, at about 03:45 BST after a Merlin Mk4 crashed.
A Royal Navy spokesperson said the deaths happened during a “helicopter training exercise”, adding “the families of the service personnel have been informed and have requested a period of grace before further details are released”.
The Princess of Wales, in her role as Commodore-in-Chief of the Fleet Air Arm, said she and William were “holding their families and friends in our hearts”.
Head of the navy, General Sir Gwyn Jenkins, confirmed that the helicopter was a Merlin Mk4, which are known to be based at RNAS Yeovilton in Somerset.
He said: “I know this will come as a huge shock to all in our naval community, and my deepest condolences go out to the families, friends and loved ones impacted by this tragedy.”
Prime Minister Sir Keir Starmer said on X that the fatal crash was “utterly tragic” and that he was thinking of “the families, friends and loved ones of the three members of the Royal Navy who sadly lost their lives”, while Defence Secretary John Healey said he was “devastated by the loss of three service personnel”.
Sarah Dyke MP for Glastonbury and Somerton said it was “desperately tragic”.
“My thoughts are with the victims and also the family of those that have lost their lives,” she added.
The Princess of Wales added she was “so saddened to hear of the tragic loss of three service personnel from the Fleet Air Arm following a helicopter training exercise last night”.
“William and I hold their families and friends in our hearts at this very difficult time,” she said.
The BBC understands that she and the Prince of Wales will be contacting the families of those who lost their lives directly in the coming days.
It is understood the King is also aware of the incident and will write privately to the families.
A spokesperson for the UK Civil Aviation Authority said the incident would be investigated by the Defence Accident Investigation Branch (DAIB).
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