The three-edged needle is used for bloodletting: the point is pricked and a few drops are expressed, chiefly for heat, stasis and acute swelling. The plum-blossom needle taps the surface repeatedly over a wider area, used for skin disorders and for superficial channel problems. The intradermal needle or press-tack is taped in place and left for hours or days, giving continuous low-grade stimulation, often in the ear or at a single body point.
All three work at the surface, and their indications are correspondingly superficial. Their advantage is that they extend treatment beyond the clinic visit, which is why the retained forms have become popular.