Three different Windows operating systems will be able to exchange files OK, although you must be sure the external drive is formatted with a file system all the computers understand. Win 98SE needs a drive formatted in FAT. Win2000 and later can use FAT or NTFS. I don’t know about Win ME.
If you really intend to synchronize files between computers two good programs are Syncback and Allway Sync. For doing backups there are many free programs to choose from. To simply move files between the computer and the external drive just use Windows Explorer.