McDormand echoed the sentiments of the night in her speech, touching on the issues of sexual harassment and gender inequality that had prompted numerous ceremony's attendees to wear black as a show of solidarity.
The film won the honor moments after McDormand won the Globe award for best actress in a film drama.
Dystopian tale "The Handmaid's Tale" won best TV drama series and "The Marvelous Mrs. Maisel", Amazon's new series about a 1950s housewife who become a stand-up comedian, took best TV comedy or musical series.
Most award winners, particularly Oprah Winfrey, Nicole Kidman and Frances McDormand, gave rousing speeches on the need for change and gender and race equality.Three Billboards., about an angry mother who goes to extremes to keep the investigation focused on her daughter's murder, became a front-runner at the Oscars.
"I forgot to thank my fellow nominees up there on the stage so I wanted to thank them and say I am honored and humbled to be in there company", said Rockwell who will next be seen with Jessica Chastain in Woman Walks Ahead. Presenting the Best Actor in a Drama award, Geena Davis said that the victor would give "half of their salary back so that women can make more than them". Also at the ceremony, Gary Oldman was named the Best Actor in a Motion Picture - Drama for his work on "Darkest Hour", a biopic about British Prime Minister Winston Churchill.More news: M&S to outsource "over half" of IT to TCS
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Moss previously won a Globe for best actress in a miniseries or TV movie for "Top of the Lake".
Women in the industry have launched the Time's Up movement to address gender disparity and abuse and nearly all of Hollywood - both women and men - turned up in black at the red carpet to show their support.
Laura Dern, a supporting actress victor for "Big Little Lies", said: "May we teach our children that speaking out without retribution is our culture's new north star". The film also took home the award for best screenplay.
After reading a quote from Atwood, she told the audience: "We no longer live in the gaps... we are the story in print and we are writing the story ourselves".
-Television, Limited Series or Motion Picture Made for TV: "Big Little Lies".