The two candidates will go head to head at the University of Nevada, 10 days after a tense second debate which saw Mr Trump threaten to jail Mrs Clinton if he was elected.
Fox News host Chris Wallace is moderating the third election clash, which will cover six topics - debt and entitlements, immigration, economy, the Supreme Court, foreign hotspots and fitness to be president.
Mr Trump is facing a battle to win over undecided voters, with multiple polls showing Mrs Clinton is leading the race to the White House.
Ahead of the third debate, the Republican nominee has faced claims of sexual misconduct from a number of women, including a former contestant on the US version of The Apprentice.
Mr Trump has insisted the allegations are “100% made up”, while his wife Melania said they were co-ordinated by his political rivals, telling Fox News: “They want to damage the presidency of my husband.”
The claims surfaced after the publication of Mr Trump’s lewd comments in a 2005 video recording, in which he talks about grabbing women “by the p***y”.
With less than three weeks remaining before Americans go to the polls, Mr Trump has claimed the November 8 election will be rigged in favour of Mrs Clinton, tweeting that “there is large-scale voter fraud happening on and before election day”.
His remarks prompted President Barack Obama to urge Mr Trump to “stop whining”, while a number of prominent Republicans have rejected claims of election rigging.
Mr Trump has also called for Mrs Clinton to take a drugs test before their final showdown after claiming she lost her energy by the end of their second election clash.
Since the last debate, Mrs Clinton has faced her own controversy following more revelations about her use of a private email server, which Mr Trump has branded a scandal “worse than Watergate”.
It emerged the US state department had asked FBI officials to lower the classification of a sensitive email related to an attack on the US compound in Benghazi, Libya.
The FBI notes revealed the discussion of a “quid pro quo” in trying to get the email reclassified. Both the state department and FBI officials have denied any bargaining took place.
The Trump campaign has said Pat Smith, whose son Sean was killed in the attack in Benghazi, will be attending the debate in Las Vegas as the Republican’s guest.
Mrs Smith was a featured speaker at the Republican National Convention in July, where she delivered an emotional speech blaming Mrs Clinton for her son’s death and accusing her of lying to families about what sparked it.
Mr Obama’s half brother Malik will also be in the audience of the third debate as a Trump supporter, telling the New York Post the Republican “can make America great again”.
The 90-minute debate starts on Thursday at 2am.