To get the SelectedValue of Dropdownlist in Edit Mode, use the below code in RowDataBound Event,
DataRowView drv = (DataRowView)e.Row.DataItem;
if (e.Row.RowType == DataControlRowType.DataRow)
{
if ((e.Row.RowState & DataControlRowState.Edit) > 0)
{
DropDownList ddl= (DropDownList )e.Row .FindControl ("DropDownList1");
ddl.SelectedValue = drv[3].ToString();
}
}