8/0 - Suitable for standard to small size tying For dry and nymphed flies 

6/0 - Stronger than 8/0, yet only a bit larger diameter. You can spin small(muddler) deer-hair & streamers with this

3/0 -   Used to tye saltwater & bass flys. It's fine diameter and strong. Wraps flat. Strong enough to spin deer-hair, or tye wool-head sculpins