James Cook, 51, son Jamie Cook, 28, and Peter Brown, 49, attacked Jason McCue in front of families and dog walkers.
The killing happened at lunchtime in John Hastie Park in Strathaven, South Lanarkshire, in July 2017.
Mr McCue, 43, was stabbed three times, including once through the heart.
Lord Matthews yesterday jailed the three for life after they were convicted of murder following a trial at the High Court in Glasgow.
The judge said the debt owed “in the scheme of things was miniscule”.
He described the killing as occurring in “the height of summer” in “full view of families enjoying their day out”.
Cook snr was ordered to served a minimum 16 years and four months, his son 17 years and three months with Brown locked up for 18 years.
Cook snr laughed and taunted Mr McCue’s emotional family as he was led handcuffed to the cells.
Jurors earlier heard how Mr McCue was set upon while in the park with a number of friends.
The court was told cash was owed to the killers. Cook snr was described as being “in charge”.
Cook jnr and Brown also went after Mr McCue’s friends James Adams, Colin Inglis and Thomas Sutherland.
But the three soon turned on Mr McCue, who ended up stricken on the ground.
Janet Guy recalled how she witnessed an attack as she walked with her grandchildren and dog.
The 69-year-old said she noticed one man with a knife and another with a hammer.
The retired hospital supervisor added: “I thought at first they were mucking about - but then realised it was more serious.
“There was a man on the ground and he was being kicked from either side.
“I panicked a bit and thought I’ve got two grandchildren here, I’ve got to get them out the park and we doubled back.”
The victim’s friend Mr Adams recalled how he was initially chased by Cook jnr - known as Cookie.
He spotted Cook snr and Brown attacking Mr McCue and Sutherland before the son joined in.
Mr Adams told police: “I didn’t see Jason move again. I definitely saw all three of them. Cookie, his dad and Peter kicking and jumping on Jason and Tam’s head.”
They escaped to the family shop - Cookie Dough - near the park. The court was yesterday told the Cooks had been “comparatively respectable” prior to the killing.