The fire department said two firefighters had died on the island of Crete “during an operation to contain a rural forest fire.”
A third firefighter was found unconscious near the Peloponnese port of Gytheio, a fire department spokesman told AFP, with media reports saying he had died of pathological causes.
State TV ERT said the two men on Crete had been trapped by the flames in their fire engine in the foothills of a mountain near the western city of Rethymno.
“Two firefighters fell in the line of duty,” deputy fire department spokesman Yiannis Artopios told AFP.
Local municipal official Sifis Vavourakis told ERT the fire front near Rethymno was over 15 kilometres long and was “out of control”.
Over 150 firefighters and eight aircraft had been dispatched, the fire department said.
But the winds complicated efforts to fight the fire from the air, local mayor Yiannis Tatarakis told ERT, adding that six villages had been evacuated.
“The wind keeps changing direction, it’s very easy to get trapped,” said Tatarakis, mayor of the village of Agios Vasileios.
With winds of up to 74 kilometres per hour, other fires were raging on the islands of Paros and Lesbos, and Messinia and Laconia in the Peloponnese peninsula.
According to local authorities, around 10 villages on Paros were evacuated on Tuesday and Wednesday, though conditions had improved later in the day.
Near the Lesbos town of Plomari, 50 firefighters, 13 engines and six aircraft were fighting a blaze that broke out near a landfill, the fire department said.
Greece faces destructive wildfires almost every summer as prolonged drought, high temperatures and strong winds create ideal conditions for blazes to spread rapidly.
The civil protection ministry placed several areas on near-maximum fire alert for Thursday, including Crete, the greater Athens area, the eastern Peloponnese and most of the Aegean islands.
On Tuesday, a 71-year-old man died of injuries from a fire outside the city of Kozani, northern Greece.
Dozens of other forest fires have broken out across the country, fuelled by strong winds. However, firefighters managed to bring them under control before they caused casualties or significant damage.
Until now, Greece had largely escaped the extreme heat and devastating wildfires that have affected several other European countries, including Spain and France this summer.
Last week, a three-day heatwave pushed temperatures to as high as 43C in some parts of the country.
According to Greece’s meteorological service, it was only the third time since 2011 that temperatures have exceeded 40C so late in the season.
