I have been in public speaking situations where I meticulously planned for a big event and when I showed up there were only three people in the audience. You can't just quit and walk away, but you can't work a plan for 500 on only 3 either.

So what do you do? You turn it into a massive positive for the three that attended. You immediately scrap your big speaking plans and tell the 3 people you will be their personal consultant for the next x hours. Then you answer their questions and make them feel like they are the only persons in the world for the entire time.

As a professional public speaker you don't mope around crying because your product sales would suffer or because your ego is suffering. You give it your all whether there are 3 or 3000 in the audience.