Mark Grieves is undertaking the challenge to raise funds for the London based children's hospital which helped care for his brother's daughter after she was born for a rare condition.
Mark's challenge begins running the Brighton marathon on April 14, before a local marathon around the track at Stoke Mandeville Stadium on his own on Saturday April 20.
Eight days later, Mark will take on the world-famous London Marathon course, before tackling the Isle of Wight challenge over the May Bank Holiday weekend, where he will run 67.2 miles in two days.
Mark took up the story of his motivation to run for Great Ormond Street.
He said: "Last year, my brother and his partner had a beautiful baby girl named Summer.
"After a pretty traumatic labour Summer was not breathing when she was born and was immediately taken to an intensive care unit.
"After being stabilised, doctors and the medical team confirmed that she was born with Pierre Robin Syndrome, which affects less than 2,000 new born children in the UK every year.
"With this news, Summer was moved to Great Ormond Street Hospital, where the medical team looked after her for the first three months of her life.
"Summer struggled to breathe, requiring a tube in one nostril to help her airways and a second tube in her other nostril to act as a feeding tube to ensure she could eat.
"The medical team spent time teaching my brother and his partner how to change the tubes and clean them in preparation for allowing Summer to go home, whilst also providing guidance, reassurance and care to two very worried parents.
"After three months in Great Ormond Street, Summer was allowed to return home.
"Over the next 12 months, Summer had weekly appointments at Great Ormond Street to check on her progress whilst having many more unplanned trips due to breathing or feeding problems.
"Summer has now been fully discharged and this wouldn’t have happened without the wonderful work of all of the staff at the hospital, who showed huge levels of patience, kindness and care throughout."
Ahead of the marathon challenge, Mark is appealing for local businesses to support his efforts by sponsoring the running tops that he will wear during the races and throughout training.
Mark is hoping for a minimum donation of £100 and in return he pledges to wear a company or organisation's logo on his running gear during training and races.
Mark is an experienced marathon runner but has never attempted so many runs in such a short space of time.
Speaking about his running experience he said: "I have run the London Marathon for the past 11 consecutive years.
"I have also run the Berlin Marathon and a few other smaller marathons over that period, along with more than 50 half marathons.
"I have always raised money for charity as part of my running, but with everything that happened with Summer when she was born, this has felt more personal, hence why I have decided to taken on such a big challenge.
"I've never ran more than two marathons in a calendar year before, so three marathons and two ultra-marathons in a month will be a huge challenge for me, but I feel really passionate about it and hopefully people feel inspired by Summers story and the challenge I am undertaking and will donate generously for Great Ormond Street."
To read more of Mark's story and to donate visit https://uk.virginmoneygiving.com/fundraiser-display/showROFundraiserPage?userUrl=markgrieves4&pageUrl=1