I see! 403 errors mean that the wrong user name and password are being sent, so the server isn't letting you connect.
I believe you'll be okay if you reset OmniFocus' keychain on all three devices, then re-enter the MobileMe login info when asked during your next sync.
You can use the link on
this page to reset OF's keychain.
If that doesn't work, contact the
support ninjas and we can gather some more information to figure out what's going on here.