The Royal trio went head-to-head in a race at the Queen Elizabeth Olympic Park in east London today.
Tensions were running high as they laced-up their trainers, stretched and got into their starting position.
But who won the 50 metre sprint?
Prince Harry pretty much left his brother and sister-in-law in the dust, but there were no hard feelings after he gave the Duke a little pat in the back.
The three joined 150 runners who are taking part in the London Marathon to raise funds for mental health campaign, Heads Together.
Athletes Iwan Thomas, Paula Radcliffe and presenter Sian Williams also took part in the relay.
Moments before the race, William joked: ‘On my hamstring’.
Kate, wearing a red puffer jacket, went full pelt but came in third.
Heads Together is an umbrella organisation fronted by the three Royals.
The charity aims to end the stigma that surrounds mental health illness.