 -- As a new father, Prince William used this Father’s Day to reflect on what he's learned about fatherhood.

William, the third in line to the throne who turns 34 on Tuesday, has two children -- Prince George, who celebrates his third birthday on July 22, and Princess Charlotte, who just turned one. William used his profile to encourage fathers from all walks of life to bring mental health issues with their children out in the open.

In a message on the website of Heads Together, a mental health campaign, William wrote: "Today I celebrate my third Father's Day as a father. For me it is a day not just to celebrate how fortunate I am for my young family, but to reflect on just how much I've learned about fatherhood and the issues facing fathers in all walks of life."

William said Prince George and Princess Charlotte's mental health is just as much of a priority as their physical health.

"In particular, it is a time to reflect on my responsibility to look after not just the physical health of my two children, but to treat their mental needs as just as important a priority," he wrote.

Prince William, Princess Kate and Prince Harry last month launched the Heads Together Campaign hoping to create a safe environment where kids feel comfortable opening up about the mental health challenges they face growing up.

The royal trio said they hope their new campaign will draw attention to the most difficult issues young people and parents face tackling bullying, suicide, homophobia and other unresolved mental health concerns.

With the support of their Royal Foundation, William, Kate and Harry will each draw attention to a different component of mental health. Prince William, the Duke of Cambridge, will raise the profile on bullying, cyber bullying and suicide among young men. Prince Harry is focusing on "invisible injuries" many veterans face after they return from the battlefield. The Duchess of Cambridge will spearhead the initiative on challenges children and young people suffer.

"So many of the issues that adolescents and adults are dealing with can be linked to unresolved childhood challenges," the Duke of Cambridge said on the website.

Prince William used Father’s Day to remind parents that safeguarding children's mental health is just as important as their physical health.

"I have been really disheartened to learn that even with all the progress made in recent years, many parents would still be ashamed if their children had a mental health problem," William wrote on the Heads Together website.

He said he is trying to end the stigma around mental health and encourage fathers to take an active role in their children's life and not be afraid to reach out for help.

"We know that fathers find asking for help harder than mothers," he wrote in his message.

The Heads Together campaign will be the single biggest initiative Prince William, Princess Kate and Prince Harry will undertake over the next year and they hope to change the conversation on mental health.

"So on this Father's Day, I encourage all fathers to take a moment to ask their children how they are doing. And know that if your son or daughter ever needs help, they need their father's guidance and support just as much as they need their mother's," he wrote.