I assume that you really want to modify the TITLE tag and not the filename.
Also, I assume that you have the correct numbering stored in the TRACK tag. IF not, try the numbering assistant first.
If you want to add a string to a tag field, have a look at the FAqs:
/t/967/1
if you want to have a three-digit number at the end of a title, then in that action from the FAQs use the string
%title% $num(%track%,3)
(or $num(%track%,2) for a 2-digit number)
Please note, that unless you have an audiobook with loads of titles that really have the same string (as there are no chapters), most players work much better with a filled track tag instead of a number at the end of a title.
Oh yes: if you want to remove the "Part" word from your titles, use an action to simply replace it with nothing.
If you want to add "part" and the track number, then append the format string and write "part" in front of $num(%track%,3).