The group had swum 300ft from the shore off Southsea near The Pyramids when they struggled to get back to the beach at 5.40pm yesterday.
Members of the public raised the alarm with nearby lifeguards who sprang into action and pulled the girls ashore.
RNLI lifeboat crews were also called and arrived within four minutes.
They wrapped the three girls in thermal plastic covers to keep them warm until ambulance staff arrived and checked them for hypothermia.
The girls were uninjured but lifeboat crews have today warned swimmers to be aware of the cold sea temperatures at this time of the year.
Coastguards say the sea temperature is 12C (53F). At the height of summer it can reach 17C (62F).
Swimmers can quickly get into difficulty because water conducts heat away from the body 23 times quicker than air.
Rescuers said the girls involved in yesterday's rescue had lost body heat during the time it took them to swim back to shore.
John Feltham, spokesman from the Portsmouth RNLI, said it was important that people were cautious about swimming in the sea at this time of year.
He said: 'The water temperature is still very low at the moment. People should not be fooled by the short spell of hot weather we are having at the moment.
'The further you venture out from the shore into the sea, the greater the risk of hypothermia. The long swim back to shore could prove fatal as you continue to lose body heat.
'They put themselves at risk by going a long way out to sea.'
Anyone who sees a swimmer in trouble should call 999 and ask for the coastguard.