See Page Numbers in Word 2007-2016 and Numbering Front Matter (Ribbon).
Page numbers are fields in headers/footers. You will want to uncheck the "Link to previous" settings in the headers/footers for your second and third sections.
Remove the page numbers from the second section by going into the header/footer that contains them and deleting them. Then go to the third section and format page numbering to start at the number you want for that section.

This is not completely dynamic. If your section one expands the page number will not change in your section 3. If you need something more dynamic, it can be done. Write back. The advantage to doing it this way is that the page numbers will be accurately reflected in a Table of Contents.