how did you identified the 'response no longer required' comes from other two tasks?

I would say it comes from first one and the other two still waits for confirmation.

if you hover over single tasks with mouse, what message do you get?

do you send those 3 tasks to the same group(s) of people?

can you post configuration of single tasks

I identified that the response no longer required came from the other two tasks by hovering over the tasks in the status screen. The images below show the messages I get from hovering over.

I currently send them all to myself only for testing purposes as the workflow is not live yet. 

Below is the configuration for one of them, they are identical except for the task name.

that's really quite strange.

I tried to setup your scenario on my own but couldn't reproduce your issue, whatever I tried.

any chance that you have somewhere in not presented workflow part a complete task action?

or a workflow on task list?

if you check workflow history/details, how many tasks do you see created there?

There is one other flexi task that occurs before the snippet shown

here is the task list from that workflow

and the task history

just to be sure, can you check that every single task you see in workflow history is really different entry with different task ID in workflow task list?

are you able to answer as well my previous questions whether you do not have any complete task action somewhere in the workflow or any workflow directly on task list?  or any other workflow within environment that could possibly cause closing of tasks?

can you post how you have configured task notifications and not required notifications?

would you be able to test with different users for different approvals?

They all have different task IDs

I do not have any complete task actions in any workflow in the farm. 

It does however appear to work if I send the approvals to different users. 

what about configuraion of both notifications?

I wonder whether if you have had them configured and then copy&pasted task action, they do not cause problems.

are you able to remove one of the task actions and add it and configure completely anew?

or to try add one another branch with same actions (configure them manually, do not copy&paste them)?

I guess this is the issue with the issue with the parallel actions. I came across the situation when the execution of one branch stops the execution of others.Check this post https://community.nintex.com/thread/12958-parallel-action-filer-condition