SDE (Software Development Engineer) - In this role, you would primarily be involved in developing software products or certain features for a product.

SDET (Software Development Engineer in Test) - In this role, you would be involved in automating test cases for the features that have been developed by SDEs.

QA tester - This role would involve manual testing of the software being developed. This won't involve much coding.

Note that both the above roles involve coding but in the case of SDE the coding is to develop an application/feature and in the case of SDET roles the code written is to automate the testing for the said features.

Let's see what activities would entail for each role

SDE (Software Development Engineer)

1. Initially, you will receive a set of requirements usually from a product manager that need to be developed.
2. You will go through the requirements keeping in mind how these can be implemented and clarify any questions you have about product behaviour. edge cases, etc. This way you will finalize the requirements along with your product manager. Note that, other people in your team, your manager, etc can be involved in these discussions.
3. Post this, there would be some amount of planning and timeline discussions with the manager involved, how many resources are needed, breaking down the features into tasks, estimating each task, etc.
4. Now, you will start the actual work on the feature, beginning with the design, you will come up with the APIs needed, how the database will look, class diagrams, use cases, any design concerns related to performance, etc.
5. Once the design has been finalized, you will actually start the implementation.
6. After you have coded the required part, you will be getting this code reviewed by your teammates or other necessary stakeholders. The code will be reviewed from both functional and quality points of view.
7. Once your code has been reviewed and approved, it will be tested by QA or whatever is applicable in your org.
8. During the testing phase, if any bugs are discovered you would be fixing these bugs as well.

SDET (Software Development Engineer in Test) and QA

1. You will also be going through requirements in order to come up with the test cases for the feature. The test cases are written from the users' perspective.
2. These test cases will also be reviewed by various stakeholders.
3. In the case of SDET, you would be automating these test cases wherever possible with the automation framework available. This will involve some amount of coding.
4. As a QA, once the feature is ready you will be manually testing the different functionalities involved. Raise bugs wherever necessary.
5. As SDET, it will be your responsibility to maintain the automation framework.
6. You will be required to evaluate with some amount of certainty your confidence in the future and raise any concerns that you have discovered as a part of the proces.