One can use weighted Random to achieve this. Declare and name a Global Random element.
SIML Code
<Siml>
<Random Name="random-question">
<Item Weight="0.2">what are you doing ?</Item>
<Item Weight="0.3">What are you upto ?</Item>
<Item Weight="0.5"></Item>
</Random>
</Siml>
And later use the random sequence in your SIML Model.
SIML Code
<Model>
<Pattern>WHAT IS YOUR NAME</Pattern>
<Response>My name is Louis. <Random Get="random-question"/></Response>
</Model>
I am assuming you are using Chatbot Studio 1.2 or above.