Help with Track Numbers

Hello everyone! I have a bunch of really old audiobooks that have no track numbers on the files at all and so they play out of order. I can see how to get to the field to put down an expression to add track numbers, I just don't know what that expression would be. Would some kind soul please help me out? I just want them numbered 1, 2, 3, 4, etc. Some audiobooks have hundreds of files, if tracks need to have leading zeros to play in order, then in the alternative, I would need an expression to add track numbers 001, 002, 003, etc. Thank you!

It is not quite clear if you just want to fill the tag-field TRACK or if you already have filled this tag-field and want to put the tracknummer to the filename.

For filling the Tag-field use the autonumbering wizard in tools.
You have to load all the files of an audiobook into MP3Tag, sort them manually according to their correct sequence, mark them, go to Tools -> Tracknumbering Wizard, check "leading zeros", and apply the tracknumberung wizard.
To sort the files manually mark them, press ALT during the action and move them to their sequence-location.

For adding the tracknumber to the filenames take the Converter Tag -> Filename and use $num(%track%,2) as format-string. You will see the immediade preview of your format string. You can add other tags to the filename like
$num(%track%,2) - %album% or
$num(%track%,2) - %album% - %title%

If 3 digits are needed, use

Actually, I would use those 3-digit track numbers only for the filename but not for the field TRACK itself as .... the Windows Explorer interprets 3-digit numbers with a leading zero as octal numbers which may lead to a strange sorting order.