Is Montmartre worth half a day if I only have 3 days in Paris?

Definitely, but don't go just for Sacré-Cœur and immediately leave.

Go early, see the basilica and the view, then spend a couple of hours walking through the quieter streets away from Place du Tertre.

Montmartre feels completely different from the monumental Paris around the Louvre and Eiffel Tower, which is exactly why I'd include it in a short itinerary.