"Lethal Weapon" won three of four film categories Sunday night at the 20th annual Image Awards presented by the National Assn. for the Advancement of Colored People. "Weapon" took best picture; Danny Glover (who co-starred with Mel Gibson in the police thriller) won best actor and Traci Wolfe, who played Glover's teen-age daughter, was named best supporting actress. Mario Van Peebles walked away with a best supporting actor award for his role in the Clint Eastwood war movie, "Heartbreak Ridge." There were no other nominations in the movie category. The awards, held this year at the Wiltern Theatre, are given annually for performances or concepts that portray a positive image of blacks.