The US presidential debates are over with Donald Trump and Hillary Clinton facing off against each other for the third and final time.

The debate, moderated by Fox News anchor Chris Wallace, saw the two presidential hopefuls exchange barbs in a feisty face off, but without a handshake exchanged between them.

It was Trump's final chance to prove to Americans that he has the temperament and ability to win the White House race.

It was also a chance for Clinton to appeal to younger voters who still see her rival as a chance for change.

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The 90-minute debate which covered the economy, social issues and foreign policy among other issues pitted the businessman against the politically experienced Clinton.

But who really came out on top?

According to Aaron Connelly, a research fellow at the Lowy Institute for International Policy, Clinton fared better when it came to policy.

He said Trump still managed to deliver a few knocks to his rival, although stressed the real winner wouldn't emerge until polls came out in a few days time.

Connelly said this final debate was similar to the first in that it showed Trump was his own worst enemy.

Not only did he allow himself to be baited by Clinton at several points but he played into her rhetoric that he doesn't have the temperament to be Commander in Chief, he said.

Trump also didn't do himself any favours with women voters calling Clinton "a nasty lady" while talking about winding back abortion rights.

"That would have hurt him with women voters," he said.

Connelly said Trump's claim that the election was rigged and his refusal to say whether he would accept the election result also damaged his chances of closing the gap with Clinton's poll lead.

"The last time an election result wasn't accepted was in 1860 and that led to a civil war" he said.

Connelly said to suggest the system was undemocratic and would create unease with immigrants who came to the US to flee political instability.

"In the last 10 years only 35 examples of voter fraud have been found out of billions of votes cast," he said.

"It's not a problem that exists... the suggestion is damaging to democratic norms."

Connelly said Trump presented himself well at the Republican debates because he was up against several candidates and excelled at using zing and bravado.

"But speaking for more than 90 seconds, like today, and he loses that cohesion," he said.

"His comments on Mosul were incomprehensible and he got key facts wrong."

Connelly said it wasn't a great look when his rival had to explain Middle East policy.

However he said he thought the first 30 minutes of the debate was when Trump was at his best as questions surrounding the Supreme Court, abortions and gun control favoured him.

"His comments on trade though fell flat though," he said.

"Clinton also pushed him on a lot of points, causing him to combust. He just got worse and worse.

"He is around 11 points down now and this debate did little to recover that divide."

Professor Simon Jackman, chief executive officer of the United Studies Centre at the University of Sydney thought the third debate was a score for Clinton.

"She was especially powerful in baiting Trump's temper and showing that he is unfit for office," Prof Jackman said.

"Her comments that they were getting Osama bin Laden while he was filming Celebrity Apprentice would also have hurt."

Prof Jackman said this debate, like much of the campaign, became quite personal at times and didn't reflect well on either of them.

"This is the most personal and ugly campaign I have seen in my 40 years of lecturing American politics," he said.

"The debate started out civil enough but didn't last long at all."

But there was one point where Clinton really shone and that was where she said Trump was always happy to blame everybody else for what went wrong.

Republican presidential nominee Donald Trump speaks during the third presidential debate with Democratic presidential nominee. Photo / AP
Republican presidential nominee Donald Trump speaks during the third presidential debate with Democratic presidential nominee. Photo / AP

Prof Jackman said she effectively managed to put Trump in a corner and contrast what he believed against everything else the US stood for, including respecting the election outcome.

"It was very purposeful and clearly well rehearsed," he said.

"It was one of her most effective moments."

When Trump spoke of Bernie Sanders criticising Clinton, Prof Jackman said she turned that on Trump by reminding him he said the Republican nominee was one of the most dangerous people to run for office in the modern age.

"In saying that Trump delivered some good blows especially on her email scandal and her levels of trustworthiness," he said.

Prof Jackman said he felt Wallace gifted Trump questions surrounding the Supreme Court and the Republican nominee but didn't build on that enough.

"The Supreme Court questions were good for Trump because it showed he was all that stood between him and Hillary Clinton making more left leaning judicial appointments," he said.

POLL RESULTS

While Clinton is leading in the polls, and has widened her gap, just how effective this debate has been won't be known for a few days yet.

However, according to broadcaster CNN, Clinton won the third debate 52 per cent to 39 per cent for Trump.

Clinton spoke for longer, getting 41:46 minutes compared to Trump's 35:41, CNN reported.

YouGov also declared Clinton the victor in the debate at 49 per cent compared to Trump's 39 per cent.

The interview of 1503 registered voters also found 12 per cent thought it was a tie.