Your post is as clear as mud. Your attachment depicts 3 tables:
• Table 1 has borders all round
• Table 2 has no borders;
• Table 3 has a bottom border.
You refer to "bottom border on 1st and 3rd only" of Table 3, but that has a bottom border on all cells. You also say "If you zoom in closely you can see the gridline for the 2nd cell" but such zooming in is not possible with your attachment, which is only an image. When I add a bottom border to just columns 1 & 3 in a 3-column table, no border appears in column 2.

Without actually seeing the problem document, it can be difficult for anyone to diagnose the issue you're having. Can you attach a document to a post with some representative tables (delete anything sensitive)? You do this via the paperclip symbol on the 'Go Advanced' tab at the bottom of this screen.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]