The three were found guilty of murder in separate cases and sentenced to death after a trial and all appeals were exhausted, Hamas said. Two were executed by firing squad and the third, a policeman, was hanged, security sources said.
"To achieve public deterrence and block crime, the relevant authorities implemented at dawn on Tuesday execution rulings against three convicted of horrifying murders," the general prosecutor's office in Gaza said in a statement.
Hamas, which has been in control of Gaza since 2007, had been under pressure not to carry out the executions, with the European Union and United Nations joining in calls from human rights groups for the decision to be set aside.