In the wake of the blockbuster "Avatar," Hollywood's obsession with 3-D has hit a roadblock. Paramount Pictures is using high-pressure tactics against theaters to book DreamWorks Animation's upcoming big-budget 3-D film, "How to Train Your Dragon" onto scarce 3-D screens around the country. "Dragon," opening March 26, will be going head to head against the swords-and-sandal 3-D picture "Clash of the Titans," from Warner Bros., which opens a week later, and Disney's 3-D "Alice in Wonderland," still drawing audiences and expected to remain in theaters for several more weeks.