try this
Dim i As Integer = 0
While i < DataGridView1.ColumnCount
DataGridView3.Columns.Add(DataGridView1.Columns(i).HeaderText, DataGridView1.Columns(i).HeaderText)
i += 1
End While
i = 0
While i < DataGridView2.ColumnCount
DataGridView3.Columns.Add(DataGridView1.Columns(i).HeaderText, DataGridView1.Columns(i).HeaderText)
i += 1
End While
i = 0
While i < DataGridView2.RowCount
DataGridView3.Rows.Insert(DataGridView1.Item(0, i).Value, DataGridView1.Item(1, i).Value, DataGridView1.Item(2, i).Value, DataGridView2.Item(0, i).Value, DataGridView2.Item(1, i).Value, DataGridView2.Item(2, i).Value)
i += 1
End While
then you can perform your addition