You can have the three styles in HtmlTextBox. For example:
<strong>bold</strong>, <em>italics</em> , <span style="text-decoration: underline">underling</span>
However, if you use expression, for example:
= IIF( Parameters.Parameter1.Value , "<strong>bold</strong>, <em>italics</em> , <span style="text-decoration: underline">underling</span>","some bold text ")
Then the expression cannot be evaluated and an error is thrown.