How to build test system for a web application with three development environments

Background

Our department is working for our web application - an ecommerce website. General working flow of a new feature are

1. Developer's coding
2. Deploy to test server
3. Tester0s test the test version which locates in test server
4. Test pass then commit changes to pre announcement server(let's say server B) and deploy
5. Testers test major functionality points on pre announcement server
6. Test pass then online server pull the code on server B, and deploy them

Notice that the pre env and online env are using the same repository.

Question

As a QA tester, how can I build regression test system for our application? Let's simply say JSON API regression test, assume we are using XUnit based framework with any tool can trigger http request and parse json then perform validation. In other word, should I manage three repository for those three environments? Or I might just take care of the test server?

Thanks!