Unless we delve into the strange universe of one-sided planes, then there is
no way to arrange 10 8.5x11 pages onto 3 tabloid pieces of paper without
leaving two 8.5x11" areas blank.  Publisher will put the blanks at the end
of the print job, unless you tell it to do otherwise.

Publisher DOES warn you of this when adding pages.  You should always create
pages in groups of four, just as Publisher tells you, precisely for this
reason - if you don't, Publisher adds whitespace that you didn't ask for.

Add two blank pages at the end of your file, and the blank pages will be at
the end of the booklet instead of the middle.  Or fill up two extra pages of
content.  Your decision.