1. A group of possible customers you can identify and reach.
2. A group with a problem they want to solve using your solution.
3. A group with the desire and ability to spend money to solve that problem.
Godin explains more here.
Three is a manageable and attainable number.
Perhaps it can be applied to other aspects of your life.

Write down three things for you.
Call three friends.
Do three on your to-do list.
Help three ideas grow.
Delete three that slow you down.
Select three colleagues you believe deserve a shot and offer a hand.
Identify three personal strengths you will exploit.
Eliminate three unnecessary possessions.
Flush out three thoughts that may have legs.
List three wishes you will grant this year.
What are your three things?
knealemann
image credit: bridges