ActionScript 3 can be used by installing the Adobe AIR SDK or Apache Flex SDK or as part Adobe's Animate CC product (formerly known as Flash Professional).
Adobe Animate CC is a professional software solution that can be used to create AS3 projects using visual tools - once installed, no further steps are necessary to begin creating AS3 projects.
The AIR SDK and Flex SDK can be used with command line tools or with various third party IDEs.
In addition to Adobe Animate CC, there are four other popular IDEs capable of working with AS3. These IDEs have their own instructions on how to get started.