how to filter duplicate records in multiple columns asp.net

Question

In my web application, i need to filter duplicate records in multiple columns.

I tried code:

select distinct col1,col2,col3 from collection
            
select col1,col2,col3 from collection Group By col1,col2,col3

I need to remove duplicate rows in col1,col2 and col3.
I tried with Group By keyword also but it displayed all duplicates also.

can anyone please tell how to do this.

Thank you