Greek rescuers were seeking four migrants missing after a small boat with seven people on board sank off the eastern Aegean Sea island of Farmakonissi.
The coast guard said three survivors have been pulled out of the sea alive after Thursday's accident.
Greece is at the forefront of Europe’s immigration crisis, with more than 700,000 people having crossed over so far this year from nearby Turkey, in frail boats provided by smuggling gangs charging high fees. Hundreds have drowned in the effort.
On Tuesday, a 3-year-old boy died when a boat carrying 25 migrants ran aground off the eastern Greek islet of Ro. The remaining occupants were unhurt.