A total of 12 networks in the US carried the debate live.

Comparisons are difficult between past debates, and even between the three debates of this presidential election cycle, as the broadcaster mix has changed each time.

But the total audience for debate number three totals 71.46 million viewers.

Roughly compared, that means it ranks as the third most watched presidential debate ever, behind the first Clinton-Trump stoush, which clocked more than 100 million viewers including online viewers, and the third debate between Jimmy Carter and Ronald Reagan in 1980.

Breaking down the channels, the debate was most watched on free-to-air television via the ABC network which drew 11 million viewers; that was followed by NBC with 10.4 million, CBS with 10.1 million and Fox with 6.6 million.

On cable, the most watched channel was Fox News, which drew 11.3 million viewers, ahead of CNN (8.7 million), MSNBC (5.5 million) and PBS 2.7 million.

Streaming numbers are difficult to measure because there is no central reporting agency, however the debate was carried online via YouTube, dedicated channels for major media outlets including key US newspapers and TV networks and the streaming platform Hulu.

NBC's YouTube stream of the debate, for example, drew an additional 5.8 million viewers and the PBS stream of the third debate drew an online audience of just over one million.

The first clash between the political rivals drew a television audience of 84 million in the US; inclusive of online streaming that figure set a total audience record of over 100 million viewers.

Their second debate drew just 68.8 million viewers, a drop of almost 20 per cent.