Parse the selected iTunes tracks' Song Names such that the same first Item text in "First Item - Second Item - Third Item" is removed and copied to another user-selected tag in each selected track (Album, Artist, Composer, Genre, Grouping, or Comments), leaving "Second Item - Third Item" in the Song Name. Run the script again to similarly remove and copy "Second Item" to another tag, and so on. Read Me explains more, and how to customize for different separator characters. Screenshot speaks volumes.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)