In “three-hour movie,” “three-hour” is an adjective that gives us information about the movie. You can easily describe the duration of an event this way. Note that “hour” is always singular, even though the number is three (or ten, or seven hundred.) 

I signed up for a four-hour workshop.

“Hour” can be replaced with other words like “day,” “month,” or “year.”

It’s a two-year program.

I’m going on a four-day camping trip.