The four main types of testing in Python 3 are Unit Testing, Integration Testing, Functional Testing, and Regression Testing. Unit Testing focuses on verifying the functionality of individual components of the software. Integration Testing tests the interdependencies between different modules of the application. Functional Testing ensures that the software behaves as per the specified requirements. Regression Testing checks for new errors, or regressions, in existing functionality after changes are made to the software. Python's standard library provides the unittest module for unit testing and third-party libraries like pytest and nose can be used for other types of testing.