An interior ministry statement said one man was spying for Israel. Palestinians who collaborate with Israeli intelligence are widely loathed AP reported.
The man was also charged with murder, as were the other two men hanged on Saturday.
The statement said the families of the victims could have saved the condemned men by forgiving them at the last minute, but they all refused.
Gaza's Hamas rulers have executed at least 18 men since seizing power in 2007.
The Palestinian Authority, which rules the West Bank, also has the death penalty. But President Mahmoud Abbas has not approved an execution since 2004, effectively halting the practice there.