You will build the gallery in child account (D). Then you will embed the gallery built in account (D) on to child account (B) and (C's) station's website.
To embed the gallery onto Station (B) and (C) you will click Publishing Options —>Embed On a Webpage.
If you are using WordPress, below is the WordPress embed code. Here is the link to the Audience WP Plugin if needed.
Factors to consider:
That all of your submissions will be housed in child account (A). Child accounts (B) and (C) do not have any data associated with the embedded contest.
Edits to the background image, overlay boxes, set-up, etc cannot be made to the stations embedding the contest. Essentially all of your galleries will mirror each other. Any updates that need to be made will be done through Station A's Audience account since it is "hosting" the Gallery.