The trio, who piled on the pounds by snacking on treats like pizza and picnic food, are among 17 animals from across the UK taking part in Pet Fit Club 2015, which is run by vet charity PDSA.
The three dogs, from Dundee, Edinburgh and Glasgow, are now following a new diet and exercise programme to help them lose weight.
Chocolate labrador Bobby, from Dundee, weighs 9st 13lbs (63.5kg) and is 100 per cent bigger than he should be.
Owner Marilyn Neave said: “Bobby is a good dog, he’s lovely natured but his mischievous side comes out during the summer when there are picnics around!
“You can’t let him off the lead if there is food about. He’s stolen many picnics during his time.”
Cocker Spaniel Harley, from Edinburgh, weighs about 4st 4lbs (27kg) and is nearly 60 per cent bigger than he should be.
Owner Lisa Mitchell said: “Other people call him fatty, and Chinese and foreign tourists always stop us in Princes Street and take photos because they think he looks like a panda.
“He’s so big he got stuck under a caravan while chasing rabbits recently and I had to push his bum to get him through.
“I want him to lose weight for his health and his happiness. He is spoilt but I want him to live longer.
“It’s also about educating us as a family. I need to stop feeling sorry for him as I feel guilty when I don’t feed him a treat.”
The third Scottish contestant is Kayla, a Mastiff-type dog who, at her heaviest, weighed 9st 8lbs (61kg).
She has already managed to lose a few pounds and currently weighs 9st 5lbs (59.4kg) but is still around a third bigger than she should be.
Her owner Agnes Higgins, from Glasgow, has given her pet pizza in the past.
She said: “The only thing Kayla doesn’t like to eat is peas but other than that she’ll eat pretty much anything.
“She doesn’t steal food but she does give you the puppy-dog eyes and I’ve fallen for them far too many times.
“I just want to help Kayla become as healthy as possible. I need to try and protect her and help her live for as long as possible.”
The animals are now following programmes specially tailored for them at Dundee, Edinburgh and Glasgow PDSA pet hospitals as they compete to lose the greatest amount of weight.
The winner of the competition will be announced in December.
Vet Hannah McIvor, from Glasgow Shamrock Street Pet Hospital, said: “Pet obesity levels are on the rise and it’s really important to tackle this issue head-on.
“PDSA research has found that four-out-of-five vet professionals have seen an increase in pet obesity cases in the last two years.
“Owners should follow guidelines when feeding their pets, not just guess or go on past experience.
“It sounds obvious but if our pets eat too much and don’t do enough exercise, then they will pile on the weight and risk developing serious diseases like heart disease.
“The Scottish pets in this year’s Pet Fit Club competition are taking the first step towards a healthier and happier new life.
“We will provide advice and support for all of the pets’ owners as they embark on this challenge.”