One of the teenagers rescued off the coast of Gwynedd was found face down in the water, the RNLI has said.
Two 16-year-olds and a 17-year-old were taken to hospital after being rescued from Tywyn North beach.
One of the other casualties was pulled from the water by people visiting the beach, but it not yet known how the third teen was rescued.
The three were at the beach with their family when they got into difficulty in the sea on Tuesday.
The casualty who was found face down was airlifted to Alder Hey Hospital in Liverpool.
The RNLI website says it has no lifeguards on beaches along the whole of the Gwynedd coastline.
Local councils and beach owners pay towards the cost of lifeguards' wages.
Gwynedd Council said a member of its maritime service was on duty at Tywyn and called the emergency services when they noticed the three were in trouble.
A spokesman added: "The council staff member then entered the sea to provide immediate assistance and with support from members of the public, our member of staff was able to assist two of the individuals to the shore.
"Gwynedd Council's maritime staff are responsible for patrolling approximately 301km of coastline.
"Our officers work closely with partners from the RNLI and the emergency services to ensure that the county's beaches and coastline are as safe as possible for the thousands of people who enjoy them every year."
In July 2015, a "selfless and brave" grandfather died trying to rescue two teenage girls from the sea at Tywyn.