If I get this right, You want your load more button to load 3 entries from all entries of all the subpages?
That would mean that you first get all entries of all projects and adapt the code accordingly. In another thread I have posted a method how to collect all structure fields of all pages. You can then take it from there.
The toStructure() method can only be used on a structure field, not on a collection of subpages, therefore your code fails.