The speedy Belgium forward struck twice in the first 16 minutes against fourth-placed Monaco to reach 17 league goals, five behind top scorer Kylian Mbappe of Paris Saint-Germain.
Openda then set up the third for midfielder Adrien Thomasson in the 56th minute, following a superbly timed run from midfield and a pinpoint pass.
The win moved Lens two points ahead of Marseille in France's second automatic Champions League spot.
Lens, who won their only league title in 1998, remain nine points behind runaway Ligue 1 leaders PSG.
Ten-time French champions Marseille have a strong away record and can reclaim second place if they win at improving Lyon on Sunday (Monday AEST).
Earlier on Saturday, Jonathan David's 21st goal of the season was not enough for Lille, who drew 1-1 at Auxerre.
The Canada striker twice hit the post before putting Lille ahead from the penalty spot in the 36th minute.
The goal moved David one behind Mbappe, who scored both of PSG's goals in a 2-1 win at bottom side Angers on Friday.
But Auxerre striker M'Baye Niang's penalty in the 62nd minute earned Auxerre a point, leaving them in 14th and four points clear of the relegation places.
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