B/G/N should all be able to communicate on the router, As B/G are fallback compatible with N, unless I’m misunderstanding your question, that is correct.
You have Clients using G/N and they can connect to a N network and the router will fallback for their speed. I assume your confusion is coming from your router configuration page that has something to the effect of the following:
Set Router in B/G/N
Set Router in G/N
Set Router in N Only