Difference between All Tasks & My Tasks
The main difference between All Tasks and My Tasks is the visibility of requests. All Tasks contains literally all requests assigned to all users which should be processed.
Difference between All Tasks & All Items
The main difference between All Tasks and All Items is the visibility of requests. In All Tasks section the user can see only requests which are not completed. That means that they are assigned to someone. In All Items you can see both, uncompleted and completed requests.