I've heard about a way to classify 3s that's similar to the phobic/counter-phobic axis for 6s. It's called shame and counter-shame. The idea is that ALL 3s are difficult to get to know, but some make themselves appear greater than they really are while others make themselves appear lesser. The shame 3s can easily be mistaken for 9s, but the difference is that they purposely make themselves appear less than what they really are, while they know personally they are something greater. That's the deception of shame 3s--there's more to them than they let on. Nines in contrast don't really have this dissonance--they may very well be someone special, but they don't necessarily think they are.
Counter-shame 3s I think are much more obvious. They overtly seek out attention and glory and can be mistaken for 7s and 8s, or are just really obvious 3s.