What time would a 3:45 movie start? Does the movie start at that time or do the ads come on first?

According Movie FAQs section of AMC Theaters: There are approximately 20 minutes of preshow material, including trailers, between the published showtime and the start of the feature film.

Around 15 to 20 minutes, as other reviewers have mentioned.

It should be around 15-20 minutes of ads after 3:45

That movie would start at approximately 4pm or 4:30pm

I think Donna here is about right. I'd say 15-20 min of ads first.