While at the supermarket a while back, one of the actions the clerk performed on the POS had a triplet of confirm modals on it, literally:
Are you sure you want to
<action>?Again, are you sure you want to
<action>?For the third time, are you sure you want to
<action>?
How could this have been designed better?
Instead of Yes/No answers for the second confirmation modal, provide response options that summarize what will happen for each possible response. For example, in the case of file deletion, use buttons labeled Delete file and Keep file.
This can keep the functionality similar and also make the experience simple and elegant.
You can also consider using progressive disclosure to allow users to find out more about the consequences of their command before they commit, while still keeping the text in the confirmation dialog brief enough to be easily scannable
However, try your best to use single confirmation modal
One practice that I have seen to ensure that the dangerous action is requested, but not clicked by accident, is to ask to enter the name of the action, the name of an item, or the user password to an input field. Using this way, the user is forced to show some more effort to confirm that they want to perform the action.
It depends on the action and the system, but please, never use a triple confirmation, that's just wrong and painful.
If it's a permanent action that you cannot easily undo (like close account or similar) you should definitely make sure the user doesn't accidentally perform the action by adding some friction.
You can accomplish this, for instance, by having them do something simple, but really unlikely to be accidental: type 'Delete' to delete... AND explain the consequences of the action.
"How could this have been designed better?"
By having one confirmation step, not three.
The action should have been: perform action > confirm action
Standard practice is to prevent error states and that's why you have the confirmation step; that's the error prevention.
In some systems where the action can be profoundly negative you may have a confirmation of the confirmation: perform action > confirm action > finalise action.
In no system I have come across would one have a triple confirmation. That's just bad design or an action like confirm nuclear launch codes.
In your example, one has to weigh up the consequences of the action versus the frequency. I doubt this cashier performs infrequent very negative actions. S/he will perform frequent, mildly negative actions.
Triple conformation is just bad design. Use single.
By making the action reversible, at least to a degree.
In your email app or on your PC, if you delete a file, it first gets thrown in the trashcan, that is an action of deletion but it can easily be reversed.
Then a secondary action is to empty the trashcan. This might be automated (monthly deletion) or it can be a manual bulk action. Say for every 10 times you empty the trash once, that gives 11 actions in total, or 1.1 action per item. Having 2-step verification with only 1.1 actual steps is a verygl good compromise. 10% more overhead/work, but 100% moren safety.
