What are best practices for creating and triggering various pipelines regards monorepos?
If this is translated to a monorepo that means that 1 change results
  in 3 pipeline triggers, while in some cases only 1 pipeline should be
  triggered.


I would definitely take this point into further consideration. One of the most compelling reasons for using a monorepo is that it does not create a technical separation of projects. There is only a logical separation, with the only technical distinction possibly being your versioning (unless you have a universal version number). This means, that you have no 100% guarantee that any one project does not depend on another project. Because of this, your final pipeline at some point should include all of the projects within your repository. By doing so, you are guaranteeing that a change to one project does not negatively impact any other projects in your repository (at the cost of the project with the longest time to build, publish, etc.). 

That being said, if there are some steps in your pipeline that you would like to run separately (lint, syntax, compile, etc.), most CI's should have a way to filter your repo for a specific folder/project. If it doesn't, and you still don't want to run against all projects, you may need to consider refactoring your repository into separate repos or changing CI tools to comply with your workflow.