<asp:Repeater ID="Repeater1" runat="server" DataSourceID="SQL_Soggetti" DataMember="DefaultView">
                        <HeaderTemplate>

        <table width="100%" border="1">
          <tr >
            <th>
              Clienti
            </th>
            <th>
              Per conto di...
            </th>
            <th>
              Destinatari
            </th>
              <th>
              &nbsp;
            </th>
          </tr>

      </HeaderTemplate>
      <ItemTemplate>

        <tr >
          <td>
            <asp:DropDownList  ID="DropDownList3" AppendDataBoundItems="true" runat="server" DataSourceID="SQL_clienti"  DataValueField="codiceIDE" DataTextField="soggetto" SelectedValue='<%# DataBinder.Eval(Container.DataItem, "cod_cliente") %>'><asp:ListItem Value="">Nessuno</asp:ListItem></asp:DropDownList> 
          </td>
          <td>
            <asp:DropDownList ID="DropDownList4" AppendDataBoundItems="true" runat="server" DataSourceID="SQL_clienti"  DataValueField="codiceIDE" DataTextField="soggetto" SelectedValue='<%# DataBinder.Eval(Container.DataItem, "cod_soggetto") %>'><asp:ListItem Value="">Nessuno</asp:ListItem></asp:DropDownList>
          </td>
          <td>
           <asp:DropDownList ID="DropDownList5" AppendDataBoundItems="true" runat="server" DataSourceID="SQL_clienti"  DataValueField="codiceIDE" DataTextField="soggetto" SelectedValue='<%# DataBinder.Eval(Container.DataItem, "cod_controparte") %>'><asp:ListItem Value="">Nessuno</asp:ListItem></asp:DropDownList>
          </td>
          <td><input type="button" value="prova" onclick="alert(document.getElementById ('<%=DropDownList3.ClientID %>'));"  /></td>
        </tr>

      </ItemTemplate>

<FooterTemplate>

        </table>

      </FooterTemplate>

                        </asp:Repeater>