potresti mettere l'ultima operazione in una sub a parte ed il confirm lo puoi simulare con l'utilizzo di due panel, tanto per capirsi una cosa del genere
codice:
<script runat="server">
sub ins_text( s as Object, e as EventArgs )
txt2.Text = txt1.Text
pnl1.Visible = false
pnl2.Visible = true
end sub
sub conferma( s as Object, e as EventArgs )
txt3.Text = txt1.Text
pnl1.Visible = true
pnl2.Visible = false
end sub
sub annulla( s as Object, e as EventArgs )
pnl1.Visible = true
pnl2.Visible = false
end sub
</script>
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<form id="myForm" runat="server">
<asp:textbox ID="txt1" runat="server" />
<asp:button ID="btnInvia" Text="esegui :." OnClick="ins_text" runat="server" />
<hr size="1">
<asp:panel ID="pnl1" runat="server">
<asp:textbox ID="txt2" ReadOnly="true" runat="server" />
<asp:textbox ID="txt3" ReadOnly="true" runat="server" />
</asp:panel>
<asp:panel ID="pnl2" Visible="false" runat="server">
<asp:label ID="lbl1" Text="continuo con la modifica?" runat="server" />
<hr size="1">
<asp:button ID="btnSi" OnClick="conferma" Text="continua" runat="server" />
<asp:button ID="btnNo" OnClick="annulla" Text="annulla" runat="server" />
</asp:panel>
</form>
</body>
</html>