Sharing read-write Excel 2003 files

i have a problem sharing read-write Excel files. i have networked 3
computers which need to share excel files. computer 1 uses excel 2002,
computer 2 stores excel files, and uses escel 2003. computer 3 uses excel
2003 as well (see diagram below). file sharing works well between computers
1 & 2 (although opening and closing files with no changes on computer 2 using
excel 2003 gives a save warning [updating from previous version] if the file
was last edited on computer 1 using excel 2002). the main problem is that
after computer 3 was added to network and is used to open excel files on
computer 2, files can only be opened as read-only. using the save as
command, warning The Folder Is Marked As Read-Only is displayed. networking
is working well and file paths are verified and correct. folder and file
sharing is enabled. no security and no share workbook are being used. any
solutions, hints, or suggestions??

computer 1 uses excel 2002
computer 2 uses excel 2003 - stores all excel files
computer 3 uses excel 2003