That put the two men only three percentage points behind Le Pen - on 22 percent - and trailing Macron, who consolidated his status as the frontrunner on 25 percent. Fillon slipped one point to 19 percent, level with Communist-backed Jean-Luc Melenchon.
In other scenarios, if Macron faces off Fillon in the runoff vote, he is expected to have a clearer victory by 68 against 32 percent; if he goes against Melenchon, he will win by 60 against 40 percent, according to the poll.
Le Pen has claimed that her proposals such as expelling any foreigner convicted of a crime or suspected of being radicalized would have helped prevent some of the attacks since 2015 which have claimed more than 200 lives.
French voters are looking for a leader who can turn their country around after years of sub-par growth and a wave of attacks by Islamist terrorists fueled a backlash against the political establishment.
They are sharing a stage but will be interviewed individually for 15 minutes as they seek to convince the electorate ahead of Sunday's first round vote.
After spiking earlier in the month as Melenchon's prospects rose, French bond yields have stabilized this week.
French far-right presidential candidate Marine Le Pen, who has dominated the campaign with her anti-immigration, anti-EU proposals, is appealing to her electoral base in the hopes of maintaining a shot at the run-off.
The police in southern France's Marseille city arrested two people and seized guns and bomb-making materials for planning an "imminent and violent attack" ahead of the first round of France's Presidential election.
"A video intercepted on 12 April showed a machinegun placed on a table, with a daily newspaper from 13 March which had on its front page one of the presidential candidates".
Authorities say they have made hundreds of arrests since the beginning of a year ago, but Tuesday's incident was the first with a possible link to the hotly contested election campaign.
Centrist Emmanuel Macron called for national unity and stronger intelligence. "As if all this was old-fashioned".
The implosion of the ruling Socialist Party, with outgoing President Francois Hollande too unpopular to run again, and the stunning success of his former economy minister, Emmanuel Macron, with an upstart middle-way grassroots campaign without major party backing, threatened to dismantle postwar France's traditional left-right political divide.
"These two radicalised men. intended to commit in the very short-term - by that I mean in the coming days - an attack on French soil", Interior Minister Matthias Fekl told a news conference. "The mission of a head of state is to guarantee your security, and I am ready for that key mission".
While prosecutors haven't identified the potential targets, Le Pen said on BFM television that "we are all targets". "Vile and unworthy statements", Macron countered.