Money raised from the event will go towards helping Macmillan Cancer Relief.
Sylvia Lewis, Macmillan's fundraising manager for Merseyside, said: "Macmillan is delighted to have been chosen as one of the charities benefiting from this year's Sefton Triathlon.
"I hope that many people choose to run, cycle and swim to raise money to help people living with cancer."
John Marsden, co-ordinator for triathlon backer Active City, said: "The triathlon is a fantastic event in that it encourages exercise in a fun way."
Olympic rowing legend Sir Steve Redgrave was at the triathlon launch at Alder Hey hospital on Friday. The triplets also met Olympeans Steve Parry and Steve Smith.
Sir Steve said: "Events like this triathlon are important in that they allow children to participate in different kinds of activity."
