Tuesday's attack, which has not been claimed, was condemned by UN chief Ban Ki-moon, who "hopes the perpetrators are swiftly identified and brought to justice," his spokesman Martin Nesirky said.
French Foreign Minister Alain Juppe meanwhile warned such attacks would not be tolerated.
"A five-vehicle convoy was between Beirut and Dar Kifa when one vehicle was hit by an explosion, the reason for which is still undetermined," said a statement from staff headquarters in Paris.
"Three French soldiers were wounded and taken to a civilian hospital in Sidon. Three others with hearing problems caused by the explosion were treated on the spot and did not require hospital treatment," it said.
Earlier, Neeraj Singh, spokesman for the United Nations Interim Force in Lebanon, told AFP: "According to preliminary reports, at around 6:00 pm today (1500 GMT) an explosion targeted a UNIFIL convoy along the highway at Sidon.
"Five UNIFIL peacekeepers were injured in the explosion. Three of the injured were transported to hospital for treatment."
One soldier was in serious condition with burns to his face and shrapnel in one eye, an official at Hammoud Hospital in Sidon told AFP.
The second soldier was slightly wounded in his left eye, the official added, and the third was released after treatment and sent back to base.
An army spokesman told AFP the bomb had been placed on the side of the road and was triggered as the convoy reached the southern entrance of Sidon.
The front of the troop carrier bore the brunt of the blast and was badly damaged, with several parts blown 20 to 30 metres (yards) away.
Several French peacekeepers could be seen covered in dust near the site of the explosion.
UNIFIL forensic experts rushed to the scene along with Lebanese troops, who cordoned off the area and began gathering evidence.
"We are working in coordination with the Lebanese army to determine the circumstances of the incident," Singh said.
Prime Minister Najib Mikati denounced the attack as immoral, and said he had asked for a speedy probe to determine who was behind it.
In Paris, Juppe condemned the attack with the "greatest firmness" and called on the Lebanese authorities to "doing everything necessary to bring the perpetrators to justice."
He said France would "not tolerate" attacks on peacekeepers," and that the "security and freedom of movement of UNIFIL soldiers must be guaranteed."
In May, six Italian peacekeepers were wounded in Sidon in a similar roadside bombing.
Nobody claimed responsibility for that attack, which targeted a UNIFIL jeep on the main highway linking the capital to south Lebanon, where the 12,000-strong force is deployed.
Spain currently commands UNIFIL, which was founded in 1978 and is tasked with overseeing a fragile peace along Lebanon's southern border with Israel.
After Italy, France forms the largest contingent with 1,600 soldiers. It is followed by Spain, with 1,100.
UNIFIL was expanded after a devastating 2006 war between the Jewish state and the Lebanese Shiite militant group Hezbollah.
The force has been the target of three other unclaimed attacks.
In the worst, three Spanish and three Colombian peacekeepers were killed in June 2007 when a booby-trapped car exploded as their vehicle passed.