The easiest way I've found to make 3D icicles for decorations, is to dip clear or irridescent threads or fibers in glue, then in glitter, to make icicles of all sizes. Using this method and Angelina heat set fibers, which are thin strands of iridescent material, I can make icicles less than 1/4 inch long, small enough to fit on N scale railroad displays or micro miniature (1:144 scale) buildings. You can see some of these tiny icicles above.
For edible 3D icicles, dipping damp (not wet!) transparent noodles (rice noodles) in sugar glitter then allowing them to dry will give you a similar effect. If the sugar glitter doesn't stick to the damp noodle, dipping the noodle into a bit of sugar syrup or white (clear) corn syrup will give you a similar effect. You can also dip dried strands of royal icing brushed with corn syrup into sugar glitter to make 3 dimensional edible icicles.