The men, dressed in red prison jumpsuits, were shackled hand and foot and hooded as they were put to death.
The men, dressed in red prison jumpsuits, were shackled hand and foot and hooded as they were put to death at an exercise ground inside a police facility, an AFP correspondent reported.
Senior Hamas officials, representatives of other Gaza-based groups and a few journalists observed the hangings.
Hamas says that the Israeli intelligence agency Mossad and its Palestinian "collaborators" killed Mazen Faqha in the Palestinian territory on March 24, but has offered no evidence.
Israel and Hamas have fought three wars in the Gaza Strip since the group wrested power from its Fatah rival in bloody fighting a decade ago.
Israel has killed several Hamas leaders in the past and has accused Mazen Faqha of being behind several deadly bombing attacks against Israelis during the second Palestinian intifada, or uprising, of 2000-2005.
Hams says that Faqha formed units of the Islamist group's military wing in the West Bank cities of Tubas, where he was born, and Jenin.
The men who were hanged on Thursday were not implicated in his killing but were accused of past acts of "treason and collaborating", a Hamas interior ministry statement said.
"The military court in Gaza sentenced them to death by hanging, and the decision was upheld by each military appeals court and the high military court," it said.
They were aged 55, 42 and 31, the ministry said but did not name them.
The Islamist group has pledged "radical measures" against Palestinians who "collaborated" with Israel.
Hamas has offered "collaborators" a brief chance to turn themselves in and receive clemency.
"The doors of repentance will be open for one week, from Tuesday, April 4 to Tuesday, April 11," the interior ministry said on Tuesday.
Hamas also tightly restricted movement out of the enclave following the assassination.
The measure remains in place despite calls from NGOs and human rights groups to lift it.
The restrictions have stopped male patients aged between 15 and 45 from using the territory's sole crossing for people to enter Israel to receive medical treatment, Human Rights Watch said.
Security checks and searches have increased, including roadblocks.
The territory has been under an Israeli blockade for the past 10 years.