Democratic nominee Hillary Clinton won Wednesday night's final presidential debate, while Republican rival Donald Trump refused to say that he would accept the result of the November 8 election if he lost.

"I will look at it at the time," Trump said when asked by moderator Chris Wallace of Fox News whether he would concede if he loses, following his claims that the election was "rigged" against him, CNN reported.

"I will keep you in suspense," he added

In response, Clinton, said her competitor's remarks were "horrifying" and accused him of taking refuge in the idea that any event that turns out against him -- even an Emmy award that goes to a rival -- was "rigged".

"That is not the way our democracy works," Clinton said, adding "We've been around for 240 years. We have had free and fair elections, accepted the outcomes when we may not have liked them. And that is what is expected of anyone standing on a debate stage during a general election."

"He is denigrating...he's talking down our democracy," the former Secretary of State added.

According to a CNN/ORC instant poll of debate watchers, Clinton beat Trump 52 per cent to 39 per cent. There was a plus or minus 4 percentage point margin of error.

Trump did outperform expectations, however. Nearly six in 10 viewers said he did better than expected, whereas 44 per cent said the same of Clinton, the poll showed.

The showdown began with the two candidates -- Clinton dressed in an off-white signature pantsuit and Trump in a black suit with a contrasting bright red tie -- taking the stage at University of Nevada's Thomas and Mack Centre without a hand-shake nor a smile.

They debated over issues like, Supreme Court; Second Amendment (protects the rights of people to keep and bear arms) where he hit Clinton for being anti-gun; abortion and the economy where the Manhattan billionaire boasted that as president, he would make the US an economic engine with annual growth rates of 5 or 6 per cent, while if Hillary goes to the White House, growth rates will be less than zero.

Trump went on to slam Clinton over her record as Secretary of State, the controversy over her private email server, and painting her as the symptom of a tired political establishment who had achieved nothing in her 30 years in public life.

Clinton she responded by going through her timeline, comparing where she was to where Trump was.

While she was in the Situation Room helping to take out slain Al Qaeda leader Osama Bin Laden in 2011, "he was hosting the Celebrity Apprentice", she said.

However, the debate took a turn when Trump and Clinton clashed over the Republican nominee's relationship with Russian President Vladimir Putin.

Clinton blasted Trump as a "puppet" of Putin and directly called on him to condemn what she said was a Russian effort to use cyberattacks to influence the election in her opponent's favour.

"That's because he (Putin) would rather have a puppet as president of the US," Clinton said, implying that Putin wanted Trump to win the election.

Trump replied that Putin had no respect for Clinton or President Barack Obama.

The Republican nominee said he had never met Putin but agreed that the Russian leader had said nice things about him, and said it would be good if Washington and Moscow worked together to fight the Islamic State (IS) which he said was "born" because of Clinton.

The two nominees also sparred over who was qualified to be President. Moderator Wallace pressed Trump on why so many women had come forward to accuse him of sexual assault if the allegations were not true.

Trump said the claims were "largely debunked". He went on to say that he did not even apologise after the 2005 tape with the lewd comments was leaked because "I did not do anything wrong".

"I think they want either fame or her campaign did it," Trump said.

In response, Clinton said, "Donald thinks belittling women makes him bigger. He goes after their dignity and their self worth."

Regarding immigration, Trump reiterated his stance to build the wall along the Mexican border.

"We all want the wall...We have to have strong borders. We have to keep drugs out of the country...She wants to have open borders," he implied.

"We have some bad 'hombres' here, and we're going to get them out," he added.

The former First Lady condemned Trump's "deportation force", which she described as impractical and "an idea that is not in keeping with who we are as a nation".

Trump, in the course of the debate also called Clinton a "nasty woman".

On closing the debate, Clinton said her main area of focus is she was elected would be education, children and family welfare; will work for everyone which will help grow the economy; and will provide protection against powerful corporations.

For his part, Trump stood firmly with his campaign one-liner 'Make America Great Again', will take care of veterans, deport all illegal aliens, more for African-Americans and Latinos; and more employment.

The third and final presidential debate was the last face-to-face interaction the two candidates will have before presidential elections next month.