Hmm, I just realized that I may have misunderstood your first
question. If the problem is that you want to keep the headers of the
first two sections blank, so that they do not reflect any insertions
made in section 3, you should use this code to unlink them:

docWord.Sections(1).Headers(1).LinkToPrevious = False
docWord.Sections(2).Headers(1).LinkToPrevious = False
docWord.Sections(3).Headers(1).LinkToPrevious = False

Then try to insert page numbers with your original code.

Note that each section actually has three different headers (and
footers); you may want to unlink those too, using Headers(2),
Headers(3) (and similarly for the footers). They won't display,
however, unless you enable the  DifferentFirstPageHeaderFooter and
OddAndEvenPagesHeaderFooter properties.