Yep. My second and third fields were empty intead of null. When I made them null, I get the same results as you. This works:

INDEX ON f1+NVL(f2,'')+NVL(f3,'') TAG f1

>Could this have anything to do with the fact that two of the index fields could be NULL? That is, the join for two of the tables is a LEFT OUTER JOIN which means the records without a matching join will contain NULLs. That's the one thing I do notice when I test the supposed blank fields, they have NULLs in them.
>
>Which makes them sort ike:
>
>AAA NULL NULL
>CCC NULL NULL
>AAA AAA AAA
>BBB AAA BBB
>CCC CCC CCC
>
>Hmmmmmmmmmmmmmm???????????????????????????????