In a Microflow you can retrieve the relevant “Car” entity and the “History” entity. Then you can add an action to change the car entity. Select the feedback attribute and fill it with $Car/ManagerFeedback + $Car/HeadFeedback + SalesManFeedback. That should do the trick, if I understand your question correctly.
You’ll need to use String Concatenation to join the 3 feedback strings together. You can then store that single string in your History entity.
Hope this helps
Easy to do with "Microflow"