third-party jar should be under <ReadyAPI installation>\bin\ext as ReadyAPI team recommended. It looks like Java project with reference library.
Generally, I did not suggest you to put any jar in /ext since it's hard to manage them as well as version control if you have more node machine to run ReadyAPI test cases. Surely, there are workaround to deploy your jar to specified folder. i.e Ansible
If you run test cases through Jenkins, you'd better to install ReadAPI in execution machine and then mount them to Jenkins as "slave" (Secondary), one more suggestion, you can configure a library with groovy as a outside script library and bind to your ReadyAPI project, then you are able to manage them with version control.
some of custom package are required to put under /ext. i.e jdbc.jar, or your encapsulated jar (i.e login.jar, assertion.jar). However, you need to consider how to manage them in your ReadyAPI project and easily to push to other machine if any update.