Barry McGivern, Scott Fryer and Troy Lacey were travelling to work at Dereham along the A47 Acle Straight, near Great Yarmouth, Norfolk.
They jumped into cold chest-high water to free the man from the crushed car and helped to resuscitate him.
Mr Lacey, 23, said: "I'm just glad we could give him a fighting chance to see his family again."
The victim is in a stable but critical condition at the Norfolk and Norwich University Hospital, police said.
The men stopped when they saw what they thought was a broken-down car and some people running along the road at about 21:00 BST on Saturday.
A blaring horn then alerted them to a car upside down in a dyke where they could see a man struggling inside, said Mr Fryer.
The unnamed victim, believed to be in his 40s, was trapped by his seatbelt in the crushed car and had his head in water.
Mr McGivern, 28, of Great Yarmouth, said he was the first to enter the "freezing cold" water which was chest-high.
Mr Lacey, from Martham, said their experience as security staff helped them to keep calm and they had freed the victim as a paramedic arrived.
"The car was that crushed that at one point I didn't think we would get him out, but I managed to get under his legs and get them free and Barry snapped the seatbelt," he said.
"His pulse was slow so I pumped his chest a few times."
Mr Fryer, also 28, a retained fire-fighter at Gorleston, then gave the victim the kiss-of-life.
"I'm just pleased we were there and we could help a fellow human being who was dying in front of us," said Mr Lacey.
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