you can handle it at c# side
Create drop down like this
<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem Text="C - Corporates" Value="C"></asp:ListItem>
<asp:ListItem Text="FT - Fasttrack" Value="FT"></asp:ListItem>
<asp:ListItem Text="Nt - NoramlTrack" Value="Nt"></asp:ListItem>
</asp:DropDownList>
Whichever fields are selected, make it comma seprated in c# with for loop or with any logic.
string s = "";
for (int i = 0; i < DropDownList1.Items.Count; i++)
{
if (DropDownList1.Items[i].Selected)
s = s + DropDownList1.Items[i].Value + ",";
}
Pass that comma seprated string to database and store it.