In the first incident, Tawfiq Ali Sharafi, 22, was killed and four others wounded during clashes with Israeli troops who invaded the towns of Bait Lahiya and Bait Hanun in the early hours of Wednesday.
Sharafi reportedly belonged to the Izz al-Din al-Qassam Brigades, the armed wing of Hamas.
Later in the day, two Palestinian teenagers, one aged 14, were killed by Israeli occupation troops who opened fire on youths throwing stones at Israeli tanks, Aljazeera's correspondent in Gaza reported.
In Netzarim another Palestinian - aged 17 years - was killed and five others were injured.
Dozens of Israeli army tanks, armoured personnel carriers and bulldozers backed by Apache helicopters stormed the area.
Local Palestinian radio stations reported that tanks fired at least seven shells into the residential area, hitting an electrical power transmitter.
Residents said the whole area was blacked out.
Several homes have also been demolished by Israeli bulldozers.
Israel said the aim of the raid was to prevent resistance fighters from firing homemade rockets into northern Israel.
Occupation troops also stormed the West Bank town of Nablus on Wednesday morning and killed an al-Aqsa Martyrs Brigades resistance fighter, Majdi Khalifa.
Israel radio reported he was a "wanted" activist. The al-Aqsa Martyrs Brigades is the armed wing of the Fatah movement.
Israel is going after Palestinian
fighters it describes as 'wanted'
Aljazeera's correspondent reports that Khalifa was killed in what seems to be an "execution operation".
Troops stormed his house in Haifa Street in central Nablus. Sounds of firing could be heard from inside, after which troops were seen dragging his body outside, the correspondent added.
Hospital sources confirmed that Khalifa's body had three bullet wounds.
A report by an Israeli newspaper said troops had stormed the house in order to arrest another suspect and when Khalifa tried to escape, troops shot him dead.
In the West Bank town of Jenin, where the Israeli army has been active in raids since Monday, troops blew up the family home of local al-Aqsa Martyrs Brigades chief Zakaria al-Zubaidi. The troops also detained a man thought to be his aide.
Nablus, the West Bank town, was
raided on Wednesday morning
Also in Jenin on Tuesday, Israeli troops killed a mentally ill Palestinian man and detained up to 20 people.
Salah Ailalu, 47, was hit in the stomach on Tuesday by troops who saw him outside after they had declared a curfew, sources said.
The curfew was imposed on Monday as part of a raid by Israeli occupation forces which residents described as the biggest in almost a year.
Tuesday marked the fourth anniversary of the current intifada (uprising) which broke out on 28 September 2000 after then Israeli opposition leader Ariel Sharon entered al-Aqsa mosque compound in Jerusalem, triggering clashes between Palestinians and Israeli forces.
Also on Wednesday, a Palestinian rocket attack killed two Israeli children in the southern town of Sderot, medics said.
Resistance fighters are regularly
scoring hits with Qassam rockets
The director of Barzilai hospital, Emile Hai, told Reuters a baby and a young boy were declared dead on arrival. Israel Radio said the boy was four years old.
Hamas' military arm Izz al-Din al-Qassam claimed responsibility for firing the Qassam rockets and said the attack was in defiance of a big Israeli military raid into northern Gaza designed to stop rocket launching.
Medics said at least 12 people were wounded in Sderot, Reuters reported.
Palestinian fighters have fired hundreds of homemade rockets during a four-year-old uprising. Two Israelis have previously been killed in such attacks - a young boy and a man who were killed in Sderot in June.
Less than an hour after the attack on Sderot, an Israeli helicopter fired a missile at an unidentified target near the Gaza refugee camp of Jabaliya, close to the zone used to launch the rockets.
Izz al-Din al-Qassam claimed to have fired six Battar missiles at a Zionist military watchtower in the Netzarim settlement to the south of Gaza city and a mortar shell at Morag settlement to the north of Rafah.
A Qassam communiqué said that the missiles were fired on Wednesday "wounding five Zionist settlers".
The Brigades also said they fired a 'Yasin' projectile at a tank heading towards Hamuda in the north of Gaza Strip.
Meanwhile, the military wing of the popular resistance committees, Al-Nasar Salah al-Din Brigades, claimed full responsibility for blasting a Zionist tank to the east of the same refugee camp. According to Hamas, eyewitnesses reported seeing the tank on fire.