How to add indicator on a light switch to indicate the outdoor 3-way light is on?
You need one or two 3-way switches with pilot lights. These are also called 3-way switches, illuminated ON. Some have an illuminated toggle, some are decorative style and some have a separate light. These are examples (not that none of the ones shown below are actually 3 way switches)



These switches generally need a neutral wire at both switches. Conventional unlighted switches do not need a neutral. They also need three wires (other than neutral) running between the switches. 

If you only want the pilot light in one location, you still need two 3-way switches, but only the lit one needs a neutral.