remove the property, TextMode="MultiLine"
<asp:TextBox ID="TextBox3" runat="server" MaxLength="100"
OnTextChanged="TextBox3_TextChanged" Font-Size="Medium"
style="z-index: 1; left: 930px; top: 178px; position: absolute;height: 40px; width: 225px"></asp:TextBox>
if you want to use multiline check no of characters on submit using
TextBox3.Text.Length
or use regular expression validator for text box3 and use regular expression : (\s|.){0,10}$
as follows
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
ControlToValidate="TextBox3" ErrorMessage="RegularExpressionValidator"
ValidationExpression="(\s|.){0,10}$"></asp:RegularExpressionValidator>