Sector Leaders showed a mixed picture Friday, with one member showing real signs of trouble. 3D Systems (DDD) continued its high-volume rush to its 10-week line by falling 3.22, or 5%, to 59.53 in double-paced trade. That's the third high-volume loss in the past four days. It's also the second test of its 50-day line in the past few weeks. The first test, seen in late January, was another high-volume affair. This is not an attractive way to set up