Fantasy still dominates the U.S. movie box office.

Leading in ticket revenue during its opening weekend, the Dreamworks animated adventure-comedy How to Train Your Dragon handily beat the raunchy comedy Hot Tub Time Machine, which was also in its opening weekend. Dragon brought in a healthy though not monumental or unexpected $43.3 million, while Time Machine came in below expectations, finishing third with only $13.7 million.

The number 1 film of the previous three weeks, Alice in Wonderland, dropped to second place with a $17.3 million box office take. But don’t feel too sorry for the filmmakers—it has already swagged $656 million worldwide in just three weeks.

A big reason for the drop in Alice attendance was . . . the debut of How to Train Your Dragon. U.S. movie theaters have a total of approximately 4,000 3D screens available nationwide, which means that when a warmly anticipated new 3D film appears, the reigning king of 3D gets bumped from many theaters, reducing revenues. James Cameron’s Avatar dropped out of the top 10 altogether, thanks at least in part to the shortage of 3D venues.

Despite the shortage of screens, 3D movies have led the U.S. box office for nine of the thirteen weekends of this year. With their significantly higher ticket prices, that’s to be expected—and can also be reckoned to fuel a further increase in 3D movies and installation of 3D screens in theaters across the country.

With Clash of the Titans coming to theaters this Friday, the shortage of 3D theaters looks likely to become particularly acute in the coming weeks.

If Titans pushes its way into a large number of theaters, Dragon‘s revenues will surely be affected. That may not be such a bad thing, however, as TAC”s Lars Walker notes.