They are all names of characters in the movie Stranger Than Fiction.
I'm not sure why the movie had people with math names, though. (Looking at Wikipedia, I see there were others, like the Kronecker bus and Euclid street.) For all its strangeness, it was a good movie, to be sure :-)
All three asked famous questions / offered famous prizes. (Hilbert's problems, Pascal offered a prize for the resolution about the "quadrature of a cycloid", whatever that is. And something on Google Books says that Mittag-Leffler offered a prize relating to something or other.)
Hilbert and Pascal seem to have thought at least some about "what is a proof?"
Perhaps it is not what you had in mind, but all three give interesting results if you type the phrase "(name) curve" into Google Image.
In particular, I had never encountered the "Mittag-Leffler Star" before.
As for the Hilbert Curve ... heck, a Kählerian tensor network version of it adorns the first page of pretty much all of our QSE Group's recent technical documents.
As a followup to the above, my literary-minded son confirms that Shreevata's answer ... that all three are characters in Stranger than Fiction ... is indeed the natural and correct answer.
Amazingly to me, my son solved the puzzle instantly ... evidently Stranger than Fiction left a considerable impression upon him (so I'm going to rent it).
Acknowledging that the above literary answer is best, an (almost) equally good mathematics-centric answer is that "The Pascal triangle, the Hilbert curve, and the Mittag-Leffler star are all Wikipedia pages about geometric objects."