For combining data from multiple datasets, all datasets must have same structure.
DataSet ds1 = new DataSet();
DataSet ds2 = new DataSet();
DataSet ds3 = new DataSet();
DataSet ds = new DataSet();
DataTable dt = ds1.Tables[0].Clone();
ds.Tables.Add(dt);
foreach (DataRow dr in ds1.Tables[0].Rows)
ds.Tables[0].ImportRow(dr);
foreach (DataRow dr in ds2.Tables[0].Rows)
ds.Tables[0].ImportRow(dr);
foreach (DataRow dr in ds3.Tables[0].Rows)
ds.Tables[0].ImportRow(dr);