I found the help page for the add-on in the the Wayback Machine, as it seems to be offline, the page as
https://plus.codes/spreadsheet-add-on redirects to a new homepage.
Seems the PLUSCODE function accepts (or did!) optional Length param...
- =PLUSCODE(location_range, [length])
- length is optional and sets the length of the code. If not specified, codes of length 10 will be returned. If it is specified it must be an even number less than 10, or any number between 10 and 16.
... so it seems should be able to do =PLUSCODE(myLat:mylng, 11)
As you want 3 after the + which is after 8.