The Coast Guard HH-65 Dolphin helicopter went down Thursday night while conducting search and rescue drills with a 47-foot motor lifeboat.
"This is a very difficult case because rescuers are being rescued," said Rear Adm. Manson Brown, 14th Coast Guard District commander.
There were no new developments from the overnight search for the missing crew member, said Coast Guard Petty Officer Luke Clayton.
The helicopter crashed about five miles south of Honolulu International Airport about 8:15 p.m. Thursday, according to the Coast Guard.
Witnesses aboard the lifeboat and a passing Air Force C-17 transport aircraft saw the crash.
Fire search crews have joined the search, reports CBS News affiliate KGMB TV in Honolulu.
"The Coast Guard is a pretty small service. Few people know we have fewer people than the Honolulu Fire Department," said Spokesman Lt. John Titchen. "So we are a very very tight family and the entire Coast Guard is grieving for the loss of our shipmates."
Honolulu firefighters pulled the three crew members from the water and transported them to the Queen's Medical Center, where they were pronounced dead. The names of the victims were not immediately released.
A Fire Department helicopter and two fire rescue boats joined the search at first light Friday, said Capt. Terry Seelig, spokesman for the Honolulu Fire Department.
The HH-65, manufactured by Aerospatiale, is 44 feet long and normally carries a pilot, co-pilot, flight mechanic and rescue swimmer. It is not capable of water landings.
The crash was the first Coast Guard helicopter accident in Hawaii since January 1982, when an HH-52 Sea Guard crashed into a cliff during a rescue mission in bad weather. Three people were killed.