There are plenty of ways to label "in between" hair if you buy into Andre's system, but I guess it seems more logical IMO to say - for example - that I have 3b/c hair rather than making up some whole new category for it. Of course most people will already understand you when you say you have 3a/4c hair, just like they'll understand me saying I have 3b/c hair. I guess I don't see how "3d" is even needed, except that it saves three typing characters.
And since nobody is going there I'll just say it: one cynical view is that some curlies will do anything to avoid calling themselves a "4" because of the whole "good hair"/"bad hair" syndrome, including calling themselves "3d"s. I think SOMETIMES this really is the motivation, though I'm not accusing you, the OP, of this.
Regardless of peoples' motivations in creating extra categories I do ultimately agree w/those who say people get to call their hair anything they want.