Here's why -- and what you should watch for Wednesday to try to game out their futures with us:
Why we think Cruz has his eye on 2020: Of the three on this list, Cruz is most obviously gaming for another presidential run. He created two political nonprofits that will essentially serve as a campaign-in-waiting should he decide to run and he’s made one of his top 2016 advisers now his top Senate adviser.
There's also the fact that he was the runner up in 2016. And, this election aside, Republican primary voters have tended to be kind to past runners up, from Ronald Reagan, George H.W. Bush and Bob Dole to John McCain and Mitt Romney.
Cruz got further than most runners up, earning hundreds more delegates and pushing the nominating contest months longer than it usually goes. He and his allies believe it's a sign the Texas senator tapped into a powerful grassroots base that, if it weren't for Trump, would have almost certainly made him the anti-establishment candidate accepting the nomination Thursday.
If Trump were to lose in November, they would have a chance to test that theory for years from now.
UPDATE: While addressing some of the 500-odd delegates bound to him at the convention on Wednesday, reporters there noted it sounded a lot like a campaign rally.
UPDATE No. 2: A top Republican party official told Yahoo News that they fully expect Cruz to run in 2020 if Trump wins in 2016.
What you should watch for in his speech Wednesday: Whether he endorses Trump. Recall that Cruz spent the last few weeks of the primary in a social media flame war with Trump: Trump insinuated Cruz’s father had something to do with the assassination of John F. Kennedy, and Cruz called Trump a “pathological liar” and a “narcissist” who was proud of being a “serial philanderer."
Now Cruz is one of the Republican Party's most conspicuous holdouts on endorsing Trump.. We can see pros and cons for his political future if he doesn't endorse Trump on Wednesday: He'll appear as a man sticking to his principles, but he also risks coming across as a man who is holding a grudge while the rest of the party has accepted reality.
On Wednesday, one of Cruz's long-time allies, Rep. Steve King (R-Iowa), told The Washington Post's Philip Rucker he thinks Cruz "needs to make the case to support [Trump], but I don't think he has to go so so far as a full-throated, enthusiastic, pound-on-the-podium endorsement."
Why we think Rubio's got his eye on 2020: Because he's running for reelection in the Senate. We at The Fix have argued that that staying in the Senate, instead of vanishing into the private sector, is the best way for the first-term Florida senator to keep his presidential prospects alive.
"Being out of the national spotlight is the equivalent of political death," wrote Fix Boss Chris Cillizza in June.
There are risks with the path Rubio has decided to take. His reelection is not a given, and losing his own seat would be a terrible start to trying to win the White House four years from now.
What you should watch for Wednesday: Whether he gets the crowd riled up. Rubio will be addressing the convention via a pre-recorded video, so he'll have to work that much harder to make an impression. But any sign of enthusiasm among convention goers -- the base of all party bases -- could suggest Rubio's still got it.
Why we think Walker doesn't have his eye on 2020: Because he's leaning toward running for reelection.
The Wisconsin governor has to make a choice about his political future sooner than the rest on this list. That's because it will be difficult for him to do both: If Walker wins a third term in 2018, his term would start in January 2019, right when it'd be time to gear up for a 2020 run.
According to the Associated Press, which caught up with him at the convention in Cleveland, Walker is leaning toward staying in Wisconsin:
Walker said he learned from his ill-fated run for president last year that “you can’t run for an office like that while still being governor, unless you’re willing to concede being governor, which I wasn’t. You’re just not able to be the candidate you need to be.” “If I run again in 2018, it would be to serve a full term and not to run for another office,” Walker said.
What you should watch for Wednesday: Confirmation he doesn't have 2020 on his mind. Walker's words and actions don't seem to be matching up at this convention -- he met with delegates from Iowa this week, the state where he launched his campaign and that fueled his short-lived, meteoric rise. That prompted Tim Alberta of the National Review to write that he think thinks this reelection talk is all a ruse and that Walker is indeed eyeing 2020: Walker "would be foolish to say anything else at this impossibly early stage," he wrote.
We'll get a glimpse into what Walker -- and the other politicians on this list -- are thinking about about their future right now on Wednesday night.