You can use a simple Execute Code card to solve this. Here is an example below that you can edit. Note at the end I am assuming you have a variable called "botResponse" you are using that can be displayed after this in a regular text card, so if you have a different variable name, change that at the end here:
// Array with different variants of the message you want to display
const messages = ["That's a fantastic idea!", "What a great idea!", "Brilliant idea!"];
// Choose a random index for the possible messages to reply with
const randomIndex = Math.floor(Math.random() * messages.length);
// Setting the output variable to the randomly chosen message
workflow.botResponse = messages[randomIndex];