If Castro can survive a serious and rigorous vetting process, you have to go with him. He's the only one of the three that has the potential to be elected President in 2024, even if Hillary isn't re-elected.. But you have to make sure he can handle a tough debate and be a serious candidate.
Heinrich or Kaine would basically be Al Gore.
Do you really think that matters?
Let's look at recent history.
2008: Even if Bush had an electable VP, like Hagel or Bill Frist, being Bush's VP only would have hurt their chances.
2000: Being VP probably hurt Al Gore in the 2000 election because he was too worried about morality and dignity to run on the economy.
1988: Kind of a hollow victory because Bush lost in 1992 and he had to deal with Iran Contra in 1988.
There is some upside to picking a rising star as your VP, because you elevate their profile for the next election, sure. But, if they're such a star, they could get the necessary profile to run on their own. (Obviously, Castro has problems with that because he can't win a Senate race in Texas)
On the downside, think of all the clunker VP candidates we've had recently, Ryan, Edwards, Palin, Quayle. There's more risk with those younger, more unknown candidates.
So, I would just pick a boring, but competent and well-liked person. Or, more importantly, the best person for the job of White House advisor and vague senior figure. The political stuff will take care of itself if you pick the best person for the job.