skip first 3 rows
Use a For loop instead of foreach.
for(int i=3; i < excelTb2.Rows.Count; i++) //start the loop with index 3 => Row 4
{
DataRow ex2 = exceltb2.Rows[i];
//ex2["ABC"] = ex1["ABC"]; // with skip(3) ?
//ex2["Name"] = ex1["Name"];
//ex2["ID"] = ex1["ID"];
}