In genetics, is intermediate inheritance the same as incomplete dominance?

I agree that the three you mentioned are the same thing.  

Codominance is when the offspring received a characteristic from both its parents.  An example would be if the offspring of a red flower and a white flower was white with red streaks.  Both phenotypes of the parents are present. They are COexisting.  

Things are polygenic when a characteristic is determined by more than one gene. Eye color is an example of a polygenic trait.  You can't do a simple mendelian square because there's no way to account for the multiple genes involved.

hope this is helpful!  good luck!