Twentieth Century Fox’s “Avatar” grossed $3.5 million in midnight runs from approximately 2,000 locations in a strong start for James Cameron’s sweeping 3D sci-fi fantasy.Friday matinees are also selling out. Many of the midnight runs were in 3D locations. “Avatar” has a strong shot at securing the best December bow of all time. Current record holder is Will Smith starrer “I Am Legend,” which posted an opening weekend gross of $77.2 million. That film grossed roughly $30.6 million on its first Friday, including $1.7 million in midnight grosses from roughly 1,100 theaters. “Avatar” also is expected to do big business in its day and date international launch, having already scored impressive first-day launches in a number of territories, including France. “Avatar” faces the challenge of being an original title, versus a franchise sequel.