Error 3 indicates that there might be an unsupported script step in your FileMaker Script. In the Script Editor, you may check your Script by using the "Show Compatibility" dropdown menu in the lower left corner.
See also http://help.filemaker.com/app/answers/detail/a_id/6474/ , especially the bottom of the List: "The following items have been added or changed in FileMaker 12"
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Thanks, Martin. It turns out that the following script steps worked in FM11 without specifying "No dialog", but apparently FM 12 is much more strict:
Commit Records/Requests [No dialog]
Change Password [No dialog; Old Password: $opw; New Password: $npw]