for loop for values of a particular column from a datatable od Dataset

Either name or index the column you want.

Look at my comments on your code below:

SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds,"FileName");
                
int countbtn = ds.Tables["FileName"].Rows.Count;
 
//loop through each row returned
foreach (DataRow row in ds.Tables["FileName"].Rows)
{
    //Loop through each column
    foreach (DataColumn dc in ds.Tables["FileName"].Columns)
    {
        //Pass the count and the whole row.
        GenrateBtn(countbtn, row.ToString());
    }
}

Below is what I think you meant:

SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds,"FileName");
                
int countbtn = ds.Tables["FileName"].Rows.Count;
 
//loop through each row returned
foreach (DataRow row in ds.Tables["FileName"].Rows)
{
    if(false)
    {
      //The below code is still wrong, but it makes use of the second loop
      
      //Loop through each column
      foreach (DataColumn dc in ds.Tables["FileName"].Columns)
      {
        //Pass the count and each column in the row.
        GenrateBtn(countbtn, row[dc.Name].ToString());
      }
   }
   else{
     //This follows your requirements:
     
     //Pass through the third column for each row (zero indexed)   
     GenrateBtn(countbtn, row[2].ToString());

     //OR use the column name
     GenrateBtn(countbtn, row["ThirdColumn"].ToString());
   }
}

Hope that helps

Andy