As reported on the mailing list,
I created a column family and added index on column A,B,C.
Now I insert three rows.
row1 : A=123, B=456, C=789
row2 : A=123, C=789
row3 : A=123, B=789, C=789
Now if I perform an indexed query for A=123 and B=456, both row1 and row2 are returned.