The Duke and Duchess of Cambridge and Prince Harry want to "help change the national conversation" on mental health.
They are leading a campaign called Heads Together which will be the biggest single project they have undertaken together. It will be a partnership with charities with experience in tackling stigma, raising awareness, and providing help for people with mental health challenges.
The campaign will be the 2017 Virgin Money London Marathon Charity of the Year. A spokesman said: "Through their work with young people, emergency response, homeless charities, and with veterans, their royal highnesses have seen that unresolved mental health problems lie at the heart of some of our greatest social challenges.
"They are passionate about tackling the stigma surrounding the issue. Too often, they have seen that people feel afraid to admit that they are struggling with their mental health. This fear of prejudice and judgment stops people from getting help and can destroy families and end lives.
"They want to help change the national conversation."
The spokesman said the three are "incredibly grateful to be working with charities who have achieved so much in the fight against the stigma that surrounds the issue".