Question on index column for a partitioned table

We have a range partitioned table on a column - extract_dt.  We are planning to create an index.  I have three columns - extract_dt, assoc_id, primary_gci -  in the where clause which appears very frequently.  I would like to create an index.  Should I use the extract_dt column as part of index (composite index - extract_dt, assoc_id, primary_gci) or just create an index on (composite - assoc_id, primary_gci) since the extract_dt is part of table partitioned column?