Macron now holds the lead in the polls - followed closely by Le Pen.
Socialist Mr Hamon, who has recently lost points in the polls, now finds himself tied with Mr Melenchon, candidate for the the far-left coalition "La France insoumise".
"For Macron the stakes are incredibly high", said the director of polling at Ifop, Jerome Fourquet. "Most French don't know him well at this stage and they'll be looking closely at his ability to take punches".
The televised debate, the first held before the first round of a French presidential election, may be crucial in helping viewers make up their minds. Much newer to politics than some of his rivals, Macron has never had any practice in television debates.
In addition to leading candidates Macron, Fillon and far-right leader Marine Le Pen, the list included three candidates who passed the threshold of 500 elected officials just in time: Jacques Cheminade and Philippe Poutou of small far-left parties and centrist lawmaker Jean Lasalle.
On Monday he will seek to claw back votes lost to 39-year-old Macron since the scandal broke in January by trying to shift the focus to his policy programme, including the radical spending cuts he says are France's only hope for real change.
Macron runs opposed to Le Pen on many issues and the race is a tight one.
Regarding security and defence, Macron and Le Pen have both pledged to increase the defence budget to three percent of the GDP.
With no candidate expected to win an outright majority during the first round election process, the two top contenders will go through to a decisive run-off on May 7.
While most of the focus has been on Fillon's legal woes and the disconnect with the "irreproachable" image that helped him win the nomination of the rightwing Republicans, Le Pen also goes into the election with several investigations hanging over the National Front (FN).