Learn German: Der Film war lang, fast drei Stunden. - The film was long, almost three hours.

war is the past tense of sein (to be), meaning "was". While spoken German mostly uses the spoken past tense (with "have" or "be" plus a past participle) for things that happened in the past, the verbs sein and haben (to have) are exceptions. We almost always use their simple past forms: war (was) and hatte (had).

Why doesn't lang have an ending like langer?

In German, adjectives only get an extra ending when they come directly before the noun they describe (like ein langer Film). When the adjective comes after a verb like "is" or "was", it stays in its basic dictionary form: Der Film war lang.

Could I say drei Uhr here instead of drei Stunden?

No, because they mean different things! Use Stunden (hours) when talking about a duration or how long something takes. Use Uhr (o'clock) when talking about a specific time on the clock, like Es ist drei Uhr (It is three o'clock).

Does fast mean "fast" (quick) like in English?

No, this is a "false friend"! In German, fast means "almost" or "nearly". If you want to say that something was fast or quick, you would use the word schnell.