As I recall, Bill was tasked to build study models for the Excelsior from concepts by others, and when he finished early he was allowed to make his own concept model for consideration. That's the one Nimoy picked.
Thus far I was unable to find Bill George's name linked to the production of ST III. The production credits
only list Fulmer and Keeler as model makers, but according to ILM-The Art of Special Effects
there were two more gentlemen, but Bill George's name isn't among these, either.