--- Find the rows using the select statement below. Do a Select count(*) for error checking.

select * from a_table 
where rowid in 
(select rowid from a_table 
minus 
select max(rowid) from a_table 
group by column1, column2, column3); 

-- Simply changing the select to a delete allows the removal of the dups.

delete from a_table 
where rowid in 
(select rowid from a_table 
minus 
select max(rowid) from a_table 
group by column1, column2, column3);