In another life, Lynn was a sniper in Afghanistan, Damien trained paramilitary forces in Iraq, and John worked undercover infiltrating drug cartels in Central America.
Now all three are back in action, this time fighting what they describe as a "war" against poachers in southern Africa as the killing of rhinos escalates into a crisis that threatens the survival of the species.
In 2008, less than 100 rhinos were poached in South Africa, but in recent years numbers have rocketed with nearly 1,200 killed in 2015 alone.
Faced with such slaughter, conservationists and government authorities have been desperately searching for ways to protect the animals.
Many ideas have been tried, including drones, tracking dogs, satellite imagery, DNA analysis, hidden cameras and even cutting horns off live animals before poachers can get to them.
But the killing has continued, and now military veterans from the United States, Australia and elsewhere have been drafted to bring their expertise to the uphill battle to save the rhinos.
"You have animals who are targeted by people using automatic weapons," Damien Mander, a former Australian Navy special forces officer, told AFP.
"You cannot go to the communities and ask them nicely to stop. This is a war. We are fighting a war out there."
Mander, who spent three years in Iraq, is the founder of the International Anti-Poaching Foundation, which supports rangers through training and promotes "direct action" to protect rhino.
"The only thing that is going to buy time for (conservation) initiatives is well-trained, well-equipped rangers who are willing to go every day and risk their lives," he said.
"I was programmed to destroy. I am now programmed to protect."
But the so-called "green militarization" of anti-poaching work - a term for the involvement of ex-military in conservation work - has aroused strong criticism, with some saying local men suspected of being poachers are being killed indiscriminately.
No figures are available, but some of the dead are from Mozambique, which borders on South Africa's Kruger National Park.
"It alienates local people and turns conservation areas into fortresses," said Libby Lunstrum, a professor at Canada's York University who specializes in poaching.
Even so, charity groups such as the International Fund for Animal Welfare are also tapping into battlefield expertise and hiring former US intelligence officers.
"When we are in a war context, we have no other means than to respond using a similar force," said Celine Sissler-Bienvenu, a director of IFAW.