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
So what do you do? You turn it into a massive positive the 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. And,
you should not insult a small audience by stating that you expect much more
attendees. Those "small numbers" who showed up could become your
major customers. You give it your
all whether there are 3 or 3000 in the audience.