I have a friend that is a 3D designer and wants to join the Stack Exchange network. We have seen different sites like hardware, English grammar and so on.
Is there any site appropriate for a 3D Developer?
I think with "sections" you mean the different StackExchange sites; they're other Q&A sites using the same engine that StackOverflow does.
There was a proposal for a 3D modelling StackExchange site site on Area 51, but it didn't make it past beta.
Your friend could try to join the Arts proposal, but it's unclear to me how questions on 3D modelling will be received there.