Three Palestinians in their 20s were killed and five others wounded in the Gaza Strip Friday, when an IDF artillery shell was carried into a home and set off accidentally, according to Palestinian security sources. The explosion caused heavy damage to the Beit Lahiya home and medics initially said the three were killed when an Israeli artillery shell hit the house, but the IDF denied firing artillery at the time of the incident. The army said the youths had been handling explosives that blew up accidently. Following a Kassam rocket attack Friday morning, the IDF responded with artillery fire directed toward launch sites in northern Gaza. Palestinian officials said a 54-year-old farmer working his fields in Jabalya was hit by shrapnel and later died of his wounds. The army said it was investigating the report. Terrorists fired four more Kassam rockets into the western Negev Saturday. One rocket hit near the cemetery in Sderot and the others in open territory. Security forces were sweeping the area in an effort to locate the rocket remains. Also Friday, Palestinians fired on an IDF patrol near the northern Gaza security fence. Troops later caught four unarmed Palestinians who had crossed into Israel. The four were handed over to security officials for questioning. Meanwhile in the West Bank, the IDF and representatives from the Civil Administration raided the offices of three Hamas-linked charities in Tulkarm and Nablus early Friday morning. The offices were allegedly used as a front to channel funds to terror cells. Documents and computers were seized and the offices closed. Palestinian Authority officials said IDF forces operating in the Balata refugee camp in Nablus shot and seriously wounded the brother of the Al Aksa Martyrs Brigades commander in the area Saturday, Army Radio reported. The army denied the report, saying security forces came under fire a number of times and explosives were thrown at troops, but they did not return fire. The troops captured an Islamic Jihad operative during the operation, and a Tanzim fugitive was arrested near Kalkilya, the IDF said. IDF troops fired at an approaching Palestinian ambulance, which was carrying a woman in labor early Saturday in Nablus. Troops earlier observed the ambulance picking up and dropping off passengers repeatedly, raising suspicion that it was involved in terror activity, military officials said. The driver and his assistant claimed they were beaten by soldiers, but the army said troops only checked the crew and woman to see if they were carrying any weapons or explosives. No one was hurt in the incident and a second ambulance was called to transport the woman to the hospital. In the past, the army has accused Palestinians of exploiting emergency vehicles to transport terrorists and explosives. A man from Safed who stopped to assist the driver of a disabled vehicle near the Sha'ar Binyamin industrial zone on Route 60 north of Jerusalem was stabbed Friday evening. The man, who was lightly wounded in the upper back, was taken to a hospital by Magen David Adom. The attack was apparently terror related. Police interviewed the victim and conducted searches in the area. AP contributed to this report.