Raised a question at Superuser.com which got answered in just a few seconds, see http://superuser.com/questions/488367/excel-2007-conditional-formatting-compare-3-columns-with-text-find-unique-v
The magic was achieved by using Conditional Formatting and adding a formula =COUNTIFS($A:$C;A1)<=2. Still having some issues, seems to be caused by me having imported the text from csv-files. Might be that if all had been typed in it would have worked without errors. Maybe need a newer version of Excel…