Thanks @Apple and @mebeingken for the full responses.
As always, the answer is not simple as there are so many factors.
For my particular case I think I will go with 3 SC API calls for the single page.
The data is static (like the example of an eBay listing) but dependent on the list item selected on another page so will only be loaded as needed.
The initial API call will return the location of some images to be displayed so it can get on with that whilst getting the other two API calls and as mebeingken mentioned, the responsiveness of the app needs to come before keeping the server happy 
It will also for my old brain to create and check 3 specific calls rather than one big one.