The Coastguard said the boy and two girls were paddling in a small river when they lost their footing and got swept out into the sea.
The children are believed to be from Ballycastle and aged 10, 11 and 12.
Nat Hunter, from Ballyvoy, was walking on the beach with his wife when he saw the children in the water.
Mr Hunter told BBC News NI: "We saw the children playing in the mouth of the river where it meets the sea.
"But when we came back they had got caught up in the current of the river and been pushed out to sea.
"They had started to panic and get stressed out, so I went to get the buoyancy aid up on the beach and went out to them.
"I threw the ring out to the child who was furthest away, about 50m out.
"It was two little girls and a boy, I think they were about ten."
Mr Hunter was helped by another man to bring the children to safety.
"When we left the coastguard was with them and the ambulance had just arrived," he said.
"I think they were alright, but they are bound to have been freezing and a bit shook up."
The Coastguard said they were called by a member of the public at about 20:30 BST on Wednesday.
"The current was strong and would have taken them out," said a spokesperson.
The children were all taken to the Causeway Hospital and are in a stable condition.
Three children have been rescued from the water in Ballycastle, County Antrim, after being spotted by a member of the public.