Most likely the line was created in Freehand and animated using Tweens in Flash. This rules out any involvement of 3DStroke or AE for that matter. You could create something similar in AE I suppose. Your best chance of doing that is to use Particular. 3DStroke is out of the question, I'd say, since it neither allows multiple overlap loops nor has it any built-in obscuration. Masking each element manually would be a major pain.