In the database side you can do it with the following script;
select col1+(col2 * col3) as col 4 from yourtable
But actually i could not understand if you are talking about this.
Or in the application side (i assume you have a datatable filled with the results) you can add a computed column as;
for example dt is your datatable in your dataset, and you want to add a computed column;
dt.Columns.Add("col4", GetType(Integer), "col1 + (col2 * col3)") in vb.net
dt.Columns.Add("col4", typeof(int), "col1 + (col2 * col3)") in c#