Try using Copy & Paste in Chirp itself, without bothering to export to a spreadsheet.

Download a fresh copy of the GT3 config from the radio.   Copy (as in cmd-C) the config lines from the GT3.  Change radios.   Then get a fresh download from the GT5 (The GT5 download should create a new tab in Chirp) Paste (cmd-V) the lines you want to ‘clone’ from the GT3 config onto the GT5 config page, ove-writing lines as needed.  Just highlight the starting line where you want the lines from the GT3 to start).  The  upload the modified config back into the GT5.  

It doesn’t hurt to save the original GT5 config that was just downloaded just in case you don’t get a satisfactory result and wish to restore.