Scarlet:

If you were to build your wing with the center section built solidy (not joined or with a wing rod) with all the shear webs and joiners, and then build joiner boxes for the poly section of the wing to fit into would be one way to accomplish a 3 section wing. The center portion of the wing usually will take on the heaviest loads during launch, etc. and will be stronger when built like this.

R/Gene