You are super super close
As mentioned in my previous message, you must have a way to Map back to the original Row.
What I mean is, how does Flow 3 know what Row to update?
When you did Get Items it just grabbed them and then you used the first one (unintentionally) and it updated.
So you are close
But what data is in Flow 3, that would make it have knowledge of for instance the Row Id of the original Created item?
Think of it like this
Flow 3 needs
Pick from the List, where (some column = some value) so that it points to the original Row in the List so you can update that Row specifically. Otherwise you cannot update it correctly(ever)
So for instance, you would need to Store the File Name in the List, so that in your Get Items, you could filter on the Filter name, which the Trigger in Flow 3 would have
FileNameColumn = 'IncomingDynamicPropertyNameForFileForTriggerGoesHere'
You would put something like that in your filter for the Get Items so it grabs the correct one, but it would only work if you are even storing the file name when you created the row in the List.
Make sense?
If this solves your problem please mark as such and maybe a like :-)
Cheers!