There si no easy way to modify the document. You would have to go through
each page in legal size, make the first paragraph on each page set with the
"Page Break Before" setting (So you know where each page starts). Then you
could convert to Letter size. Then you would have to find a way to place 3
inches worth of text back on the previous page. You could reduce font size,
font spacing, interline space, paragraph spacing, etc...

A big mess...

Instead, look in your "Printing" dialog (Ctrl-P). In the bottom right corner
there is a Zoom section. In that Zoom section you should have a Scaling
option. You can keep the document formatted in Legal size, but scale it down
to Letter size when printing. Keep in mind that this will create slightly
larger left/right margins as each page is shrunk to fit a a letter size page
(Letter and Legal size have the same width, just a different length).