After they leave the stage tonight, they're off on their own sprints to the finish, focused on events designed to boost their support in the last 20 days of the race.
Here are five storylines to keep in mind while watching the debate.
What Happens in Vegas Won't Stay in Vegas
Tonight Clinton and Trump will face off a final time at the University of Nevada at Las Vegas — the beginning of the end of their campaigns.
The past two debates had the largest audiences in presidential history, and all signs point to tonight's debate serving to completing a ratings hat trick.
Though they both have a bevy of campaign events between now and Election Day, they can't reach as many people at those events as they can from the debate stage.
As a result, this could be their last chance to really connect with a large number of undecided voters before ballots are cast.
The Final Face-Off
Over the course of the first two debates, the candidates have become progressively less cordial. Trump has dropped the pleasantries from the first debate, when he referred to Clinton as "Secretary Clinton," and she addressed him as "Donald." They both dispensed with the theater of the polite handshake at the start of the second debate.
Though it seems unlikely that they will return to protocol, a handshake at the start of the third debate could be an olive branch as they head into the final 20 days of campaigning.
Rhetoric About Rigging
Trump has spent much of the past few days talking about how he believes the election is "rigged," without providing evidence to support those claims.
He mentioned the "rigged" nature of the election more than 20 times over the weekend, sewing it into the latest iterations of his stump speech and making it a near certainty that he'll bring it up tonight.
Exchanges Over Email
Swing State Showdown
Nevada is one of the states in play right now. ABC News has declared the state a toss-up in the presidential race.
So while Trump and Clinton will be addressing viewers on the national level, working in some local flavor could help them win over area voters as well.